diff --git a/modules/openapi-generator/src/main/resources/python/api.mustache b/modules/openapi-generator/src/main/resources/python/api.mustache
index 4319b31edfdc..c771516a8a49 100644
--- a/modules/openapi-generator/src/main/resources/python/api.mustache
+++ b/modules/openapi-generator/src/main/resources/python/api.mustache
@@ -308,7 +308,7 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -387,12 +387,12 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
 {{#headerParams}}
         if {{paramName}} is not None:
             {{#vendorExtensions.x-python-json-header}}
-            _header_params['{{baseName}}'] = json.dumps(
+            self.api_client._set_header(_header_params, '{{baseName}}', json.dumps(
                 self.api_client.sanitize_for_serialization({{paramName}})
-            )
+            ))
             {{/vendorExtensions.x-python-json-header}}
             {{^vendorExtensions.x-python-json-header}}
-            _header_params['{{baseName}}'] = {{paramName}}
+            self.api_client._set_header(_header_params, '{{baseName}}', {{paramName}})
             {{/vendorExtensions.x-python-json-header}}
 {{/headerParams}}
         # process the form parameters
@@ -431,13 +431,13 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
         {{/isQueryParam}}
         {{#isHeaderParam}}
         # Set client side default value of Header Param "{{baseName}}".
-        _header_params['{{baseName}}'] = {{#_enum}}'{{{.}}}'{{/_enum}}
+        self.api_client._set_header(_header_params, '{{baseName}}', {{#_enum}}'{{{.}}}'{{/_enum}})
         {{/isHeaderParam}}
         {{/constantParams}}
 
         {{#hasProduces}}
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [{{#produces}}
                     '{{{mediaType}}}'{{^-last}}, {{/-last}}{{/produces}}
@@ -448,7 +448,7 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
         {{#hasConsumes}}
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -458,7 +458,7 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
         {{/hasConsumes}}
 
         # authentication setting
diff --git a/modules/openapi-generator/src/main/resources/python/api_client.mustache b/modules/openapi-generator/src/main/resources/python/api_client.mustache
index 973a8de03b2b..9e16a465bd57 100644
--- a/modules/openapi-generator/src/main/resources/python/api_client.mustache
+++ b/modules/openapi-generator/src/main/resources/python/api_client.mustache
@@ -20,7 +20,7 @@ from threading import Lock
 {{/compatibleWithPythonLegacyApi}}
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union{{#compatibleWithPythonLegacy}}, Any{{/compatibleWithPythonLegacy}}
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from {{packageName}}.configuration import Configuration
@@ -231,16 +231,36 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
 {{#useIndependentImplicitClients}}
@@ -321,10 +341,9 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -856,7 +875,11 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -865,7 +888,7 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             {{#hasHttpSignatureMethods}}
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
@@ -873,7 +896,8 @@ https://github.com/OpenAPITools/openapi-generator/blob/c84b949df1a9ec04ba75989cb
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
             {{/hasHttpSignatureMethods}}
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
diff --git a/modules/openapi-generator/src/main/resources/python/asyncio/rest.mustache b/modules/openapi-generator/src/main/resources/python/asyncio/rest.mustache
index fefd5e2af81d..220c8335c36f 100644
--- a/modules/openapi-generator/src/main/resources/python/asyncio/rest.mustache
+++ b/modules/openapi-generator/src/main/resources/python/asyncio/rest.mustache
@@ -11,6 +11,7 @@ from typing import Any, Dict, Optional, Union
 
 import aiohttp
 import aiohttp_retry
+from multidict import CIMultiDict
 
 from {{packageName}}.exceptions import ApiException, ApiValueError
 
@@ -191,7 +192,7 @@ class RESTClientObject:
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = CIMultiDict(headers or {})
         # url already contains the URL query string
         timeout = _request_timeout or 5 * 60
 
diff --git a/modules/openapi-generator/src/main/resources/python/common_README.mustache b/modules/openapi-generator/src/main/resources/python/common_README.mustache
index 0001c1be1c03..6b3344bc648f 100644
--- a/modules/openapi-generator/src/main/resources/python/common_README.mustache
+++ b/modules/openapi-generator/src/main/resources/python/common_README.mustache
@@ -108,3 +108,33 @@ Class | Method | HTTP request | Description
 
 {{#apiInfo}}{{#apis}}{{#-last}}{{infoEmail}}
 {{/-last}}{{/apis}}{{/apiInfo}}
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/modules/openapi-generator/src/main/resources/python/httpx/rest.mustache b/modules/openapi-generator/src/main/resources/python/httpx/rest.mustache
index 4cd2e513896a..749b7ffbadbf 100644
--- a/modules/openapi-generator/src/main/resources/python/httpx/rest.mustache
+++ b/modules/openapi-generator/src/main/resources/python/httpx/rest.mustache
@@ -110,7 +110,7 @@ class RESTClientObject:
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = {{httpxModule}}.Headers(headers or {})
         timeout = _request_timeout or 5 * 60
 
         if 'Content-Type' not in headers:
diff --git a/modules/openapi-generator/src/main/resources/python/rest.mustache b/modules/openapi-generator/src/main/resources/python/rest.mustache
index 5f56a2d60b32..efefcf117882 100644
--- a/modules/openapi-generator/src/main/resources/python/rest.mustache
+++ b/modules/openapi-generator/src/main/resources/python/rest.mustache
@@ -212,7 +212,7 @@ class RESTClientObject:
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java
index 4c90e38d4ab7..3a92212c882e 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java
@@ -174,9 +174,9 @@ public void testJsonContentHeaderUsesJsonSerialization() throws IOException {
         TestUtils.assertFileContains(
                 api,
                 "import json",
-                "_header_params['X-Json-Arg'] = json.dumps(",
+                "self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(",
                 "self.api_client.sanitize_for_serialization(x_json_arg)",
-                "_header_params['X-Plain-Arg'] = x_plain_arg");
+                "self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)");
 
         TestUtils.assertFileNotContains(
                 api,
@@ -806,7 +806,7 @@ public void testHandleConstantParams() throws IOException {
         File apiFile = files
                 .get(Paths.get(output.getAbsolutePath(), "openapi_client", "api", "hello_example_api.py").toString());
         assertNotNull(apiFile);
-        assertFileContains(apiFile.toPath(), "_header_params['X-CUSTOM_CONSTANT_HEADER'] = 'CONSTANT_VALUE'");
+        assertFileContains(apiFile.toPath(), "self.api_client._set_header(_header_params, 'X-CUSTOM_CONSTANT_HEADER', 'CONSTANT_VALUE')");
         assertFileContains(apiFile.toPath(), "_query_params.append(('CONSTANT_QUERY_STRING_KEY', 'CONSTANT_QUERY_STRING_VALUE'))");
     }
 
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/README.md b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/README.md
index bbdaf5e1d957..91786c45dfc1 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/README.md
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/README.md
@@ -165,3 +165,33 @@ Authentication schemes defined for the API:
 
 team@openapitools.org
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/auth_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/auth_api.py
index 3ad40309ce6f..551e07af01bd 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/auth_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/auth_api.py
@@ -236,7 +236,7 @@ def _test_auth_http_basic_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -251,7 +251,7 @@ def _test_auth_http_basic_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -482,7 +482,7 @@ def _test_auth_http_bearer_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -497,7 +497,7 @@ def _test_auth_http_bearer_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/body_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/body_api.py
index c4b05d54f131..69360616c450 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/body_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/body_api.py
@@ -241,7 +241,7 @@ def _test_binary_gif_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -256,7 +256,7 @@ def _test_binary_gif_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'image/gif'
@@ -499,7 +499,7 @@ def _test_body_application_octetstream_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -524,7 +524,7 @@ def _test_body_application_octetstream_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -533,7 +533,7 @@ def _test_body_application_octetstream_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -543,7 +543,7 @@ def _test_body_application_octetstream_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -781,7 +781,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -798,7 +798,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -807,7 +807,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -817,7 +817,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1054,7 +1054,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1071,7 +1071,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1080,7 +1080,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1090,7 +1090,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1327,7 +1327,7 @@ def _test_echo_body_all_of_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1344,7 +1344,7 @@ def _test_echo_body_all_of_pet_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1353,7 +1353,7 @@ def _test_echo_body_all_of_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1363,7 +1363,7 @@ def _test_echo_body_all_of_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1600,7 +1600,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1617,7 +1617,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1626,7 +1626,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1636,7 +1636,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1873,7 +1873,7 @@ def _test_echo_body_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1890,7 +1890,7 @@ def _test_echo_body_pet_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1899,7 +1899,7 @@ def _test_echo_body_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1909,7 +1909,7 @@ def _test_echo_body_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2146,7 +2146,7 @@ def _test_echo_body_pet_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2163,7 +2163,7 @@ def _test_echo_body_pet_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2172,7 +2172,7 @@ def _test_echo_body_pet_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2182,7 +2182,7 @@ def _test_echo_body_pet_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2419,7 +2419,7 @@ def _test_echo_body_string_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2436,7 +2436,7 @@ def _test_echo_body_string_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2445,7 +2445,7 @@ def _test_echo_body_string_enum_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2455,7 +2455,7 @@ def _test_echo_body_string_enum_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2692,7 +2692,7 @@ def _test_echo_body_tag_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2709,7 +2709,7 @@ def _test_echo_body_tag_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2718,7 +2718,7 @@ def _test_echo_body_tag_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2728,7 +2728,7 @@ def _test_echo_body_tag_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/form_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/form_api.py
index e4e3ddffa3ac..e9b0038cda16 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/form_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/form_api.py
@@ -277,7 +277,7 @@ def _test_form_integer_boolean_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -298,7 +298,7 @@ def _test_form_integer_boolean_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -307,7 +307,7 @@ def _test_form_integer_boolean_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -317,7 +317,7 @@ def _test_form_integer_boolean_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -554,7 +554,7 @@ def _test_form_object_multipart_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -571,7 +571,7 @@ def _test_form_object_multipart_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -580,7 +580,7 @@ def _test_form_object_multipart_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -590,7 +590,7 @@ def _test_form_object_multipart_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -892,7 +892,7 @@ def _test_form_oneof_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -919,7 +919,7 @@ def _test_form_oneof_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -928,7 +928,7 @@ def _test_form_oneof_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -938,7 +938,7 @@ def _test_form_oneof_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/header_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/header_api.py
index 9dbee4922feb..b7572b76afec 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/header_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/header_api.py
@@ -303,7 +303,7 @@ def _test_header_integer_boolean_string_enums_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -314,21 +314,21 @@ def _test_header_integer_boolean_string_enums_serialize(
         # process the query parameters
         # process the header parameters
         if integer_header is not None:
-            _header_params['integer_header'] = integer_header
+            self.api_client._set_header(_header_params, 'integer_header', integer_header)
         if boolean_header is not None:
-            _header_params['boolean_header'] = boolean_header
+            self.api_client._set_header(_header_params, 'boolean_header', boolean_header)
         if string_header is not None:
-            _header_params['string_header'] = string_header
+            self.api_client._set_header(_header_params, 'string_header', string_header)
         if enum_nonref_string_header is not None:
-            _header_params['enum_nonref_string_header'] = enum_nonref_string_header
+            self.api_client._set_header(_header_params, 'enum_nonref_string_header', enum_nonref_string_header)
         if enum_ref_string_header is not None:
-            _header_params['enum_ref_string_header'] = enum_ref_string_header
+            self.api_client._set_header(_header_params, 'enum_ref_string_header', enum_ref_string_header)
         # process the form parameters
         # process the body parameter
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/path_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/path_api.py
index e4a07a198eb4..d3e2de0529f9 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/path_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/path_api.py
@@ -289,7 +289,7 @@ def _tests_path_string_path_string_integer_path_integer_enum_nonref_string_path_
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -312,7 +312,7 @@ def _tests_path_string_path_string_integer_path_integer_enum_nonref_string_path_
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/query_api.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/query_api.py
index 05837d93cbee..d8271833134e 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/query_api.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api/query_api.py
@@ -267,7 +267,7 @@ def _test_enum_ref_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -290,7 +290,7 @@ def _test_enum_ref_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -559,7 +559,7 @@ def _test_query_datetime_date_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -604,7 +604,7 @@ def _test_query_datetime_date_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -873,7 +873,7 @@ def _test_query_integer_boolean_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -900,7 +900,7 @@ def _test_query_integer_boolean_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1143,7 +1143,7 @@ def _test_query_style_deep_object_explode_true_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1162,7 +1162,7 @@ def _test_query_style_deep_object_explode_true_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1405,7 +1405,7 @@ def _test_query_style_deep_object_explode_true_object_all_of_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1424,7 +1424,7 @@ def _test_query_style_deep_object_explode_true_object_all_of_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1668,7 +1668,7 @@ def _test_query_style_form_explode_false_array_integer_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1687,7 +1687,7 @@ def _test_query_style_form_explode_false_array_integer_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1931,7 +1931,7 @@ def _test_query_style_form_explode_false_array_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1950,7 +1950,7 @@ def _test_query_style_form_explode_false_array_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2193,7 +2193,7 @@ def _test_query_style_form_explode_true_array_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2212,7 +2212,7 @@ def _test_query_style_form_explode_true_array_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2455,7 +2455,7 @@ def _test_query_style_form_explode_true_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2474,7 +2474,7 @@ def _test_query_style_form_explode_true_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2717,7 +2717,7 @@ def _test_query_style_form_explode_true_object_all_of_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2736,7 +2736,7 @@ def _test_query_style_form_explode_true_object_all_of_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2993,7 +2993,7 @@ def _test_query_style_json_serialization_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3016,7 +3016,7 @@ def _test_query_style_json_serialization_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
index 77721a58cb85..516f7fb1a206 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/api_client.py
@@ -24,7 +24,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from openapi_client.configuration import Configuration
@@ -104,16 +104,36 @@ def __exit__(self, exc_type, exc_value, traceback):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -180,10 +200,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -705,7 +724,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -714,7 +737,7 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/rest.py b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/rest.py
index 9a0c6d4de785..702a9b13b966 100644
--- a/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/rest.py
+++ b/samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/openapi_client/rest.py
@@ -217,7 +217,7 @@ def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/samples/client/echo_api/python/README.md b/samples/client/echo_api/python/README.md
index bbdaf5e1d957..91786c45dfc1 100644
--- a/samples/client/echo_api/python/README.md
+++ b/samples/client/echo_api/python/README.md
@@ -165,3 +165,33 @@ Authentication schemes defined for the API:
 
 team@openapitools.org
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/client/echo_api/python/openapi_client/api/auth_api.py b/samples/client/echo_api/python/openapi_client/api/auth_api.py
index 3ad40309ce6f..551e07af01bd 100644
--- a/samples/client/echo_api/python/openapi_client/api/auth_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/auth_api.py
@@ -236,7 +236,7 @@ def _test_auth_http_basic_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -251,7 +251,7 @@ def _test_auth_http_basic_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -482,7 +482,7 @@ def _test_auth_http_bearer_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -497,7 +497,7 @@ def _test_auth_http_bearer_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python/openapi_client/api/body_api.py b/samples/client/echo_api/python/openapi_client/api/body_api.py
index c4b05d54f131..69360616c450 100644
--- a/samples/client/echo_api/python/openapi_client/api/body_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/body_api.py
@@ -241,7 +241,7 @@ def _test_binary_gif_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -256,7 +256,7 @@ def _test_binary_gif_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'image/gif'
@@ -499,7 +499,7 @@ def _test_body_application_octetstream_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -524,7 +524,7 @@ def _test_body_application_octetstream_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -533,7 +533,7 @@ def _test_body_application_octetstream_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -543,7 +543,7 @@ def _test_body_application_octetstream_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -781,7 +781,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -798,7 +798,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -807,7 +807,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -817,7 +817,7 @@ def _test_body_multipart_formdata_array_of_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1054,7 +1054,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1071,7 +1071,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1080,7 +1080,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1090,7 +1090,7 @@ def _test_body_multipart_formdata_single_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1327,7 +1327,7 @@ def _test_echo_body_all_of_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1344,7 +1344,7 @@ def _test_echo_body_all_of_pet_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1353,7 +1353,7 @@ def _test_echo_body_all_of_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1363,7 +1363,7 @@ def _test_echo_body_all_of_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1600,7 +1600,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1617,7 +1617,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1626,7 +1626,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1636,7 +1636,7 @@ def _test_echo_body_free_form_object_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1873,7 +1873,7 @@ def _test_echo_body_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1890,7 +1890,7 @@ def _test_echo_body_pet_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1899,7 +1899,7 @@ def _test_echo_body_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1909,7 +1909,7 @@ def _test_echo_body_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2146,7 +2146,7 @@ def _test_echo_body_pet_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2163,7 +2163,7 @@ def _test_echo_body_pet_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2172,7 +2172,7 @@ def _test_echo_body_pet_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2182,7 +2182,7 @@ def _test_echo_body_pet_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2419,7 +2419,7 @@ def _test_echo_body_string_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2436,7 +2436,7 @@ def _test_echo_body_string_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2445,7 +2445,7 @@ def _test_echo_body_string_enum_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2455,7 +2455,7 @@ def _test_echo_body_string_enum_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2692,7 +2692,7 @@ def _test_echo_body_tag_response_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2709,7 +2709,7 @@ def _test_echo_body_tag_response_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2718,7 +2718,7 @@ def _test_echo_body_tag_response_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2728,7 +2728,7 @@ def _test_echo_body_tag_response_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/client/echo_api/python/openapi_client/api/form_api.py b/samples/client/echo_api/python/openapi_client/api/form_api.py
index e4e3ddffa3ac..e9b0038cda16 100644
--- a/samples/client/echo_api/python/openapi_client/api/form_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/form_api.py
@@ -277,7 +277,7 @@ def _test_form_integer_boolean_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -298,7 +298,7 @@ def _test_form_integer_boolean_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -307,7 +307,7 @@ def _test_form_integer_boolean_string_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -317,7 +317,7 @@ def _test_form_integer_boolean_string_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -554,7 +554,7 @@ def _test_form_object_multipart_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -571,7 +571,7 @@ def _test_form_object_multipart_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -580,7 +580,7 @@ def _test_form_object_multipart_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -590,7 +590,7 @@ def _test_form_object_multipart_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -892,7 +892,7 @@ def _test_form_oneof_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -919,7 +919,7 @@ def _test_form_oneof_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -928,7 +928,7 @@ def _test_form_oneof_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -938,7 +938,7 @@ def _test_form_oneof_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/client/echo_api/python/openapi_client/api/header_api.py b/samples/client/echo_api/python/openapi_client/api/header_api.py
index 9dbee4922feb..b7572b76afec 100644
--- a/samples/client/echo_api/python/openapi_client/api/header_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/header_api.py
@@ -303,7 +303,7 @@ def _test_header_integer_boolean_string_enums_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -314,21 +314,21 @@ def _test_header_integer_boolean_string_enums_serialize(
         # process the query parameters
         # process the header parameters
         if integer_header is not None:
-            _header_params['integer_header'] = integer_header
+            self.api_client._set_header(_header_params, 'integer_header', integer_header)
         if boolean_header is not None:
-            _header_params['boolean_header'] = boolean_header
+            self.api_client._set_header(_header_params, 'boolean_header', boolean_header)
         if string_header is not None:
-            _header_params['string_header'] = string_header
+            self.api_client._set_header(_header_params, 'string_header', string_header)
         if enum_nonref_string_header is not None:
-            _header_params['enum_nonref_string_header'] = enum_nonref_string_header
+            self.api_client._set_header(_header_params, 'enum_nonref_string_header', enum_nonref_string_header)
         if enum_ref_string_header is not None:
-            _header_params['enum_ref_string_header'] = enum_ref_string_header
+            self.api_client._set_header(_header_params, 'enum_ref_string_header', enum_ref_string_header)
         # process the form parameters
         # process the body parameter
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python/openapi_client/api/path_api.py b/samples/client/echo_api/python/openapi_client/api/path_api.py
index e4a07a198eb4..d3e2de0529f9 100644
--- a/samples/client/echo_api/python/openapi_client/api/path_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/path_api.py
@@ -289,7 +289,7 @@ def _tests_path_string_path_string_integer_path_integer_enum_nonref_string_path_
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -312,7 +312,7 @@ def _tests_path_string_path_string_integer_path_integer_enum_nonref_string_path_
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python/openapi_client/api/query_api.py b/samples/client/echo_api/python/openapi_client/api/query_api.py
index 05837d93cbee..d8271833134e 100644
--- a/samples/client/echo_api/python/openapi_client/api/query_api.py
+++ b/samples/client/echo_api/python/openapi_client/api/query_api.py
@@ -267,7 +267,7 @@ def _test_enum_ref_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -290,7 +290,7 @@ def _test_enum_ref_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -559,7 +559,7 @@ def _test_query_datetime_date_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -604,7 +604,7 @@ def _test_query_datetime_date_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -873,7 +873,7 @@ def _test_query_integer_boolean_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -900,7 +900,7 @@ def _test_query_integer_boolean_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1143,7 +1143,7 @@ def _test_query_style_deep_object_explode_true_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1162,7 +1162,7 @@ def _test_query_style_deep_object_explode_true_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1405,7 +1405,7 @@ def _test_query_style_deep_object_explode_true_object_all_of_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1424,7 +1424,7 @@ def _test_query_style_deep_object_explode_true_object_all_of_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1668,7 +1668,7 @@ def _test_query_style_form_explode_false_array_integer_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1687,7 +1687,7 @@ def _test_query_style_form_explode_false_array_integer_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -1931,7 +1931,7 @@ def _test_query_style_form_explode_false_array_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1950,7 +1950,7 @@ def _test_query_style_form_explode_false_array_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2193,7 +2193,7 @@ def _test_query_style_form_explode_true_array_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2212,7 +2212,7 @@ def _test_query_style_form_explode_true_array_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2455,7 +2455,7 @@ def _test_query_style_form_explode_true_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2474,7 +2474,7 @@ def _test_query_style_form_explode_true_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2717,7 +2717,7 @@ def _test_query_style_form_explode_true_object_all_of_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2736,7 +2736,7 @@ def _test_query_style_form_explode_true_object_all_of_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -2993,7 +2993,7 @@ def _test_query_style_json_serialization_object_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3016,7 +3016,7 @@ def _test_query_style_json_serialization_object_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
diff --git a/samples/client/echo_api/python/openapi_client/api_client.py b/samples/client/echo_api/python/openapi_client/api_client.py
index 77721a58cb85..516f7fb1a206 100644
--- a/samples/client/echo_api/python/openapi_client/api_client.py
+++ b/samples/client/echo_api/python/openapi_client/api_client.py
@@ -24,7 +24,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from openapi_client.configuration import Configuration
@@ -104,16 +104,36 @@ def __exit__(self, exc_type, exc_value, traceback):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -180,10 +200,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -705,7 +724,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -714,7 +737,7 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/client/echo_api/python/openapi_client/rest.py b/samples/client/echo_api/python/openapi_client/rest.py
index 77b4fe56053a..3c35dc796332 100644
--- a/samples/client/echo_api/python/openapi_client/rest.py
+++ b/samples/client/echo_api/python/openapi_client/rest.py
@@ -217,7 +217,7 @@ def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/samples/client/others/python-legacy-model-dictionaries/README.md b/samples/client/others/python-legacy-model-dictionaries/README.md
index 0877f5284828..25df6323a088 100644
--- a/samples/client/others/python-legacy-model-dictionaries/README.md
+++ b/samples/client/others/python-legacy-model-dictionaries/README.md
@@ -112,3 +112,33 @@ Endpoints do not require authorization.
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api/default_api.py b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api/default_api.py
index 77aba80c25eb..2dff3238e7c7 100644
--- a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api/default_api.py
+++ b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api/default_api.py
@@ -197,7 +197,7 @@ def _list_legacy_models_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -212,7 +212,7 @@ def _list_legacy_models_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api_client.py b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api_client.py
index dffac725656d..25b5f869407f 100644
--- a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api_client.py
+++ b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/api_client.py
@@ -26,7 +26,7 @@
 from threading import Lock
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union, Any
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from legacy_model_dict_client.configuration import Configuration
@@ -208,16 +208,36 @@ def __exit__(self, exc_type, exc_value, traceback):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -284,10 +304,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -813,7 +832,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -822,7 +845,7 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/rest.py b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/rest.py
index c6da043e386b..705b48b5625c 100644
--- a/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/rest.py
+++ b/samples/client/others/python-legacy-model-dictionaries/legacy_model_dict_client/rest.py
@@ -216,7 +216,7 @@ def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/samples/openapi3/client/petstore/python-aiohttp/README.md b/samples/openapi3/client/petstore/python-aiohttp/README.md
index 3f56c38a5e94..7e15564de3cc 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/README.md
+++ b/samples/openapi3/client/petstore/python-aiohttp/README.md
@@ -348,3 +348,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/another_fake_api.py
index d840f94840b5..efe518659150 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/another_fake_api.py
@@ -269,7 +269,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -286,7 +286,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -295,7 +295,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -305,7 +305,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/default_api.py
index de0a0e9d83f5..9370d404e284 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/default_api.py
@@ -251,7 +251,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -266,7 +266,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_api.py
index 37debd4fa721..99e4031552c1 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_api.py
@@ -284,7 +284,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -303,7 +303,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -313,7 +313,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -547,7 +547,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -786,7 +786,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -801,7 +801,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1067,7 +1067,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1082,7 +1082,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1092,7 +1092,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1103,7 +1103,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1341,7 +1341,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1358,7 +1358,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1367,7 +1367,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1377,7 +1377,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1614,7 +1614,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1631,7 +1631,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1640,7 +1640,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1650,7 +1650,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1887,7 +1887,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1904,7 +1904,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1913,7 +1913,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1923,7 +1923,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2160,7 +2160,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2177,7 +2177,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2186,7 +2186,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2196,7 +2196,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2447,7 +2447,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2468,7 +2468,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2477,7 +2477,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2487,7 +2487,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2708,7 +2708,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2723,7 +2723,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -2950,7 +2950,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2965,7 +2965,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3192,7 +3192,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3207,7 +3207,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3434,7 +3434,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3449,7 +3449,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3676,7 +3676,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3691,7 +3691,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3918,7 +3918,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3933,7 +3933,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4160,7 +4160,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4175,7 +4175,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4402,7 +4402,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4417,7 +4417,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4644,7 +4644,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4659,7 +4659,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4886,7 +4886,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4901,7 +4901,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5141,7 +5141,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5396,7 +5396,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5415,7 +5415,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5425,7 +5425,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5662,7 +5662,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5689,7 +5689,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5699,7 +5699,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5936,7 +5936,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5955,7 +5955,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5965,7 +5965,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6212,7 +6212,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6235,7 +6235,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6245,7 +6245,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6482,7 +6482,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6499,7 +6499,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6508,7 +6508,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6518,7 +6518,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6765,7 +6765,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7023,7 +7023,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7038,7 +7038,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -7466,7 +7466,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7513,7 +7513,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7523,7 +7523,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7751,7 +7751,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7766,7 +7766,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8074,7 +8074,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8101,9 +8101,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -8346,7 +8346,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8365,7 +8365,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8375,7 +8375,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8612,7 +8612,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8631,7 +8631,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8641,7 +8641,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8891,7 +8891,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8912,7 +8912,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8922,7 +8922,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9169,7 +9169,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9180,11 +9180,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -9423,7 +9423,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9442,7 +9442,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9452,7 +9452,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9772,7 +9772,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10051,7 +10051,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10070,7 +10070,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10080,7 +10080,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10343,7 +10343,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10364,7 +10364,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -10373,7 +10373,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10383,7 +10383,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_classname_tags123_api.py
index 3bdf586ba12d..61dbccb2cc61 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/fake_classname_tags123_api.py
@@ -269,7 +269,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -286,7 +286,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -295,7 +295,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -305,7 +305,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/import_test_datetime_api.py
index d2470196f063..3556c26f0a22 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/import_test_datetime_api.py
@@ -251,7 +251,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -266,7 +266,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/pet_api.py
index 8f290212ddbb..6aec11242f59 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/pet_api.py
@@ -274,7 +274,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -293,7 +293,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -304,7 +304,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -559,7 +559,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -572,7 +572,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -819,7 +819,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -838,7 +838,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1091,7 +1091,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1110,7 +1110,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1362,7 +1362,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1379,7 +1379,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1633,7 +1633,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1652,7 +1652,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1663,7 +1663,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1931,7 +1931,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1954,7 +1954,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1964,7 +1964,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2228,7 +2228,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2249,7 +2249,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2258,7 +2258,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2268,7 +2268,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2532,7 +2532,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2553,7 +2553,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2562,7 +2562,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2572,7 +2572,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/store_api.py
index e47aba3dc9f0..85a52d796667 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/store_api.py
@@ -273,7 +273,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -513,7 +513,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -528,7 +528,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -778,7 +778,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -795,7 +795,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1042,7 +1042,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1059,7 +1059,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1069,7 +1069,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1079,7 +1079,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/user_api.py
index 1246baedd369..a6ed97e521c4 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api/user_api.py
@@ -273,7 +273,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -292,7 +292,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -302,7 +302,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -537,7 +537,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -556,7 +556,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -566,7 +566,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -801,7 +801,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -820,7 +820,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -830,7 +830,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1070,7 +1070,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1329,7 +1329,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1346,7 +1346,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1606,7 +1606,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1629,7 +1629,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1857,7 +1857,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2124,7 +2124,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2145,7 +2145,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2155,7 +2155,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
index 50741490c29c..249166feb5ba 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ async def close(self):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -179,10 +199,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -704,7 +723,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -713,14 +736,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/rest.py b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/rest.py
index 374aeeb993fa..f9c8d3b2bab2 100644
--- a/samples/openapi3/client/petstore/python-aiohttp/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-aiohttp/petstore_api/rest.py
@@ -20,6 +20,7 @@
 
 import aiohttp
 import aiohttp_retry
+from multidict import CIMultiDict
 
 from petstore_api.exceptions import ApiException, ApiValueError
 
@@ -193,7 +194,7 @@ async def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = CIMultiDict(headers or {})
         # url already contains the URL query string
         timeout = _request_timeout or 5 * 60
 
diff --git a/samples/openapi3/client/petstore/python-aiohttp/tests/test_request_headers.py b/samples/openapi3/client/petstore/python-aiohttp/tests/test_request_headers.py
new file mode 100644
index 000000000000..f59bf5a64c49
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-aiohttp/tests/test_request_headers.py
@@ -0,0 +1,228 @@
+"""Request-header behavior for the asyncio client."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import inspect
+import unittest
+from typing import Any
+from unittest.mock import AsyncMock, Mock
+
+import petstore_api
+
+ASYNC = inspect.iscoroutinefunction(petstore_api.ApiClient.call_api)
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[dict[str, Any]] = []
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        response = Mock(status=200, reason='OK', data=b'{}', headers={})
+        response.read = AsyncMock(return_value=b'{}')
+
+        def respond(*args, **kwargs):
+            self.requests.append({**kwargs, 'headers': kwargs['headers'].copy()})
+            return response
+
+        async def respond_async(*args, **kwargs):
+            await asyncio.sleep(0)
+            return respond(*args, **kwargs)
+
+        self.pool: Any = AsyncMock() if ASYNC else Mock()
+        self.pool.request.side_effect = respond_async if ASYNC else respond
+        self.client.rest_client.pool_manager = self.pool
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        if ASYNC:
+            await getattr(self.client, 'close')()
+        else:
+            self.pool.clear()
+
+    async def call(self, **kwargs):
+        result = self.api.add_pet(self.pet, **kwargs)
+        if inspect.isawaitable(result):
+            await result
+
+    def values(self, headers, name):
+        if isinstance(headers, dict):
+            return [value for key, value in headers.items() if key.lower() == name.lower()]
+        return headers.getall(name) if ASYNC else headers.getlist(name)
+
+    def assert_header(self, name, value):
+        self.assertEqual(self.values(self.requests[-1]['headers'], name), [value])
+
+    async def test_generated_content_type_and_accept_are_case_insensitive(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'custom'}
+                    await self.call(_headers=headers)
+                    self.assert_header('content-type', 'application/json')
+                    self.assert_header('accept', 'custom')
+                    self.assertEqual(headers, {spelling: value, 'accept': 'custom'})
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.call()
+        self.assert_header('user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_content_type_override_and_defaults_retain_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.call(_headers=headers, _content_type='application/custom+json')
+        self.assert_header('content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.call(_headers=headers, _content_type='application/custom+json')
+        self.assert_header('content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_last_write_and_user_agent_property(self):
+        self.client.set_default_header('X-Default', 'old')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'new')
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.call(_headers=headers)
+        self.assert_header('x-default', 'new')
+        self.assert_header('user-agent', 'custom')
+        self.assert_header('x-list', 'a, b, a')
+        self.assertEqual(headers['X-List'], 'first')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_serializers_preserve_inputs_and_explicit_header_wins(self):
+        headers = {'content-type': 'text/plain', 'API_KEY': 'call'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.client.param_serialize('GET', '/pet', header_params=headers, query_params=[])
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_authentication_and_empty_override(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        headers = {'authorization': 'call'}
+        defaults = self.client.default_headers.copy()
+        await self.call(_headers=headers, _request_auth={})
+        self.assert_header('authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.call(_headers=headers, _request_auth=auth)
+        self.assert_header('authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.client.set_default_header('API_KEY', 'default')
+        key_headers = {'Api_Key': 'call'}
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=key_headers, auth_settings=['api_key'], query_params=[]
+        )
+        result = self.client.call_api(*params)
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('api_key', 'configured-key')
+        self.assertEqual(key_headers, {'Api_Key': 'call'})
+
+    async def test_cookie_authentication_appends_without_empty_separator(self):
+        for value, expected in (('', 'session=two'), ('client=1', 'client=1; session=two')):
+            headers = {'cookie': value}
+            params = self.client.param_serialize(
+                'GET', '/cookie', header_params=headers, query_params=[],
+                auth_settings=['cookie'],
+                _request_auth={
+                    'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two',
+                },
+            )
+            result = self.client.call_api(*params)
+            if inspect.isawaitable(result):
+                await result
+            self.assert_header('cookie', expected)
+            self.assertEqual(headers, {'cookie': value})
+
+    async def test_concurrent_calls_keep_inputs_defaults_and_credentials_isolated(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def auth(index):
+            return {'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                    'value': f'Bearer request-{index}'}
+
+        if ASYNC:
+            await asyncio.gather(*(
+                self.call(_headers=headers, _request_auth=auth(index)) for index in range(20)
+            ))
+        else:
+            def run_calls():
+                with ThreadPoolExecutor(max_workers=4) as executor:
+                    list(executor.map(lambda index: self.api.add_pet(
+                        self.pet, _headers=headers, _request_auth=auth(index)
+                    ), range(20)))
+            await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request['headers']['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        await self.call()
+        self.assert_header('authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_transport_reads_lowercase_content_type(self):
+        headers = {'content-type': 'application/x-www-form-urlencoded'}
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('content-type', 'application/x-www-form-urlencoded')
+        import aiohttp
+        self.assertIsInstance(self.requests[-1]['data'], aiohttp.FormData)
+        self.assertEqual(headers, {'content-type': 'application/x-www-form-urlencoded'})
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        from multidict import CIMultiDict
+        headers: Any = CIMultiDict([
+            ('content-type', 'application/x-www-form-urlencoded'),
+            ('x-list', 'a'), ('x-list', 'b'),
+        ])
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('content-type', 'application/x-www-form-urlencoded')
+        self.assertEqual(self.values(self.requests[-1]['headers'], 'x-list'), ['a', 'b'])
+        self.assertEqual(self.values(headers, 'x-list'), ['a', 'b'])
+        import aiohttp
+        self.assertIsInstance(self.requests[-1]['data'], aiohttp.FormData)
+
+    async def test_transport_multipart_removes_content_type_without_mutating_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assertNotIn('content-type', self.requests[-1]['headers'])
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/README.md b/samples/openapi3/client/petstore/python-httpx-sync/README.md
index b69b28fb19fc..d696bf8659b7 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/README.md
+++ b/samples/openapi3/client/petstore/python-httpx-sync/README.md
@@ -326,3 +326,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/another_fake_api.py
index 3c8c561664d3..3e70d5e8c536 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/another_fake_api.py
@@ -353,7 +353,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -370,7 +370,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -379,7 +379,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -389,7 +389,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/default_api.py
index 227cb6c9bf89..0044d1e0287c 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/default_api.py
@@ -329,7 +329,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -344,7 +344,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_api.py
index 05c6c53603af..4a6f7b7183da 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_api.py
@@ -368,7 +368,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -387,7 +387,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -397,7 +397,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -733,7 +733,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1068,7 +1068,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1083,7 +1083,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1463,7 +1463,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1478,7 +1478,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1488,7 +1488,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1499,7 +1499,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1839,7 +1839,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1856,7 +1856,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1865,7 +1865,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1875,7 +1875,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2214,7 +2214,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2231,7 +2231,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2240,7 +2240,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2250,7 +2250,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2589,7 +2589,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2606,7 +2606,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2615,7 +2615,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2625,7 +2625,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2964,7 +2964,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2981,7 +2981,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2990,7 +2990,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3000,7 +3000,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3359,7 +3359,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3380,7 +3380,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -3389,7 +3389,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3399,7 +3399,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3716,7 +3716,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3731,7 +3731,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4054,7 +4054,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4069,7 +4069,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4392,7 +4392,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4407,7 +4407,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4730,7 +4730,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4745,7 +4745,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5068,7 +5068,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5083,7 +5083,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -5406,7 +5406,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5421,7 +5421,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5744,7 +5744,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5759,7 +5759,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6082,7 +6082,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6097,7 +6097,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6420,7 +6420,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6435,7 +6435,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -6758,7 +6758,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6773,7 +6773,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -7115,7 +7115,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7472,7 +7472,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7491,7 +7491,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7501,7 +7501,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7840,7 +7840,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7867,7 +7867,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7877,7 +7877,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8216,7 +8216,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8235,7 +8235,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8245,7 +8245,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8600,7 +8600,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8623,7 +8623,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8633,7 +8633,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8972,7 +8972,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8989,7 +8989,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8998,7 +8998,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9008,7 +9008,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9363,7 +9363,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9717,7 +9717,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9732,7 +9732,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -10346,7 +10346,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10393,7 +10393,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10403,7 +10403,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10727,7 +10727,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10742,7 +10742,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -11182,7 +11182,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11209,9 +11209,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -11556,7 +11556,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11575,7 +11575,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -11585,7 +11585,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -11924,7 +11924,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11943,7 +11943,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -11953,7 +11953,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -12311,7 +12311,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -12332,7 +12332,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -12342,7 +12342,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -12697,7 +12697,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -12708,11 +12708,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -13053,7 +13053,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13072,7 +13072,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -13082,7 +13082,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -13540,7 +13540,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13921,7 +13921,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13940,7 +13940,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -13950,7 +13950,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -14327,7 +14327,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -14348,7 +14348,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -14357,7 +14357,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -14367,7 +14367,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_classname_tags123_api.py
index 3ae5963332d5..da4d328c100f 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/fake_classname_tags123_api.py
@@ -353,7 +353,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -370,7 +370,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -379,7 +379,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -389,7 +389,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/import_test_datetime_api.py
index cf3b1e092de7..fd933e5180e0 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/import_test_datetime_api.py
@@ -329,7 +329,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -344,7 +344,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/pet_api.py
index 4228f8f36727..d304e3fba145 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/pet_api.py
@@ -358,7 +358,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -377,7 +377,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -388,7 +388,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -751,7 +751,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -764,7 +764,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -1113,7 +1113,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1132,7 +1132,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1487,7 +1487,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1506,7 +1506,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1860,7 +1860,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1877,7 +1877,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2233,7 +2233,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2252,7 +2252,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2263,7 +2263,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2645,7 +2645,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2668,7 +2668,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2678,7 +2678,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3056,7 +3056,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3077,7 +3077,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3086,7 +3086,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3096,7 +3096,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3474,7 +3474,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3495,7 +3495,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3504,7 +3504,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3514,7 +3514,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/store_api.py
index 3829184bb1fe..b6c56870658d 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/store_api.py
@@ -357,7 +357,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -693,7 +693,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -708,7 +708,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1060,7 +1060,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1077,7 +1077,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1426,7 +1426,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1443,7 +1443,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1453,7 +1453,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1463,7 +1463,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/user_api.py
index 85d34b6ebb62..d07e94b739fe 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api/user_api.py
@@ -357,7 +357,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -376,7 +376,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -386,7 +386,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -723,7 +723,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -742,7 +742,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -752,7 +752,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1089,7 +1089,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1108,7 +1108,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1118,7 +1118,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1460,7 +1460,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1821,7 +1821,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1838,7 +1838,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2206,7 +2206,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2229,7 +2229,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2553,7 +2553,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2928,7 +2928,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2949,7 +2949,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2959,7 +2959,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api_client.py
index 44b7a286135f..43782b4ec85b 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ async def close(self):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -182,10 +202,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -707,7 +726,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -716,14 +739,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/rest.py b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/rest.py
index e1d0094b6c56..f28745ec0fe7 100644
--- a/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-httpx-sync/petstore_api/rest.py
@@ -119,7 +119,7 @@ async def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = httpx.Headers(headers or {})
         timeout = _request_timeout or 5 * 60
 
         if 'Content-Type' not in headers:
diff --git a/samples/openapi3/client/petstore/python-httpx-sync/tests/test_request_headers.py b/samples/openapi3/client/petstore/python-httpx-sync/tests/test_request_headers.py
new file mode 100644
index 000000000000..addec9e857d8
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-httpx-sync/tests/test_request_headers.py
@@ -0,0 +1,247 @@
+"""Regression tests for request-local, case-insensitive HTTPX headers."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import importlib
+import unittest
+from typing import Any
+
+import petstore_api
+from petstore_api import rest
+
+httpx = importlib.import_module(rest.RESTResponseType.__module__.split('.')[0])
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[Any] = []
+
+        async def respond(request):
+            self.requests.append(request)
+            await asyncio.sleep(0)
+            return httpx.Response(200, json={})
+
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        self.client.rest_client.pool_manager = httpx.AsyncClient(
+            transport=httpx.MockTransport(respond)
+        )
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        await self.client.close()
+
+    def assert_single_header(self, request, name, value):
+        self.assertEqual(request.headers.get_list(name), [value])
+
+    async def test_serializers_copy_header_inputs_before_adding_defaults_and_auth(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(headers, original)
+        self.client.param_serialize(
+            'GET', '/pet', header_params=headers, query_params=[], auth_settings=['petstore_auth']
+        )
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_generated_content_type_replaces_case_variants(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'application/custom'}
+                    original = headers.copy()
+                    await self.api.add_pet(self.pet, _headers=headers)
+                    self.assert_single_header(
+                        self.requests[-1], 'content-type', 'application/json'
+                    )
+                    self.assert_single_header(self.requests[-1], 'accept', 'application/custom')
+                    self.assertEqual(headers, original)
+
+    async def test_content_type_override_and_client_default_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_and_last_inserted_case_variant_win_without_splitting_values(self):
+        self.client.set_default_header('X-Default', 'earlier-default')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'default')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-default', 'default')
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b, a')
+        self.assertIn('x-list', self.requests[-1].headers)
+        self.assertEqual(headers['X-List'], 'first')
+
+    async def test_user_agent_default_updates_case_insensitively(self):
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'custom')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_empty_request_auth_keeps_configured_authentication(self):
+        await self.api.add_pet(self.pet, _request_auth={})
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+
+    async def test_empty_cookie_does_not_add_leading_separator(self):
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params={'cookie': ''}, query_params=[],
+            auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'session=two')
+
+    async def test_explicit_header_parameter_overrides_call_headers(self):
+        headers = {'API_KEY': 'call'}
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.assertEqual(headers, {'API_KEY': 'call'})
+
+    async def test_authentication_overrides_case_variants_and_preserves_inputs(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        defaults = self.client.default_headers.copy()
+        headers = {'authorization': 'call'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.api.add_pet(self.pet, _headers=headers, _request_auth=auth)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_api_key_overrides_default_and_call_case_variants(self):
+        self.client.set_default_header('API_KEY', 'default')
+        headers = {'Api_Key': 'call'}
+        defaults = self.client.default_headers.copy()
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=headers, auth_settings=['api_key'], query_params=[]
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'api_key', 'configured-key')
+        self.assertEqual(headers, {'Api_Key': 'call'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_cookie_authentication_appends_to_selected_cookie(self):
+        self.client.cookie = 'client=1'
+        headers = {'cookie': 'call=1'}
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params=headers, query_params=[], auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'client=1; session=two')
+        self.assertEqual(headers, {'cookie': 'call=1'})
+
+    async def test_concurrent_calls_do_not_share_headers_or_authentication(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        async def call(index):
+            await self.api.add_pet(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer request-{index}',
+            })
+
+        await asyncio.gather(*(call(index) for index in range(20)))
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_public_mapping_validation_retains_combined_values(self):
+        headers = httpx.Headers([('X-List', 'a'), ('x-list', 'b')])
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b')
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        headers = httpx.Headers([
+            ('content-type', 'application/x-www-form-urlencoded'),
+            ('x-list', 'a'), ('x-list', 'b'),
+        ])
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        self.assert_single_header(
+            self.requests[-1], 'content-type', 'application/x-www-form-urlencoded'
+        )
+        self.assertEqual(self.requests[-1].content, b'field=value')
+        self.assertEqual(self.requests[-1].headers.get_list('x-list'), ['a', 'b'])
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_multipart_does_not_mutate_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        self.assertTrue(
+            self.requests[-1].headers['content-type'].startswith('multipart/form-data;')
+        )
+        self.assertEqual(len(self.requests[-1].headers.get_list('content-type')), 1)
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})
+
+    async def test_sync_calls_use_request_local_headers(self):
+        if not hasattr(self.api, 'add_pet_sync'):
+            self.skipTest('Sample does not enable supportHttpxSync')
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def call(index):
+            getattr(self.api, 'add_pet_sync')(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer sync-{index}',
+            })
+
+        def run_calls():
+            with ThreadPoolExecutor(max_workers=4) as executor:
+                list(executor.map(call, range(8)))
+
+        await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer sync-{index}' for index in range(8)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
diff --git a/samples/openapi3/client/petstore/python-httpx/README.md b/samples/openapi3/client/petstore/python-httpx/README.md
index b69b28fb19fc..d696bf8659b7 100644
--- a/samples/openapi3/client/petstore/python-httpx/README.md
+++ b/samples/openapi3/client/petstore/python-httpx/README.md
@@ -326,3 +326,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/another_fake_api.py
index 9ceb307bbb24..968ac5d448d0 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/another_fake_api.py
@@ -250,7 +250,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/default_api.py
index dc264f69428e..5a766861f81f 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/default_api.py
@@ -232,7 +232,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_api.py
index 7fb0a185a264..8c7dc59ccaac 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_api.py
@@ -265,7 +265,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -284,7 +284,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -294,7 +294,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -528,7 +528,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -767,7 +767,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -782,7 +782,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1048,7 +1048,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1063,7 +1063,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1073,7 +1073,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1084,7 +1084,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1322,7 +1322,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1339,7 +1339,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1348,7 +1348,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1358,7 +1358,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1595,7 +1595,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1612,7 +1612,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1621,7 +1621,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1631,7 +1631,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1868,7 +1868,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1885,7 +1885,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1894,7 +1894,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1904,7 +1904,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2141,7 +2141,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2158,7 +2158,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2167,7 +2167,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2177,7 +2177,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2428,7 +2428,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2449,7 +2449,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2458,7 +2458,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2468,7 +2468,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2689,7 +2689,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2704,7 +2704,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -2931,7 +2931,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2946,7 +2946,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3173,7 +3173,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3188,7 +3188,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3415,7 +3415,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3430,7 +3430,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3657,7 +3657,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3672,7 +3672,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3899,7 +3899,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3914,7 +3914,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4141,7 +4141,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4156,7 +4156,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4383,7 +4383,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4398,7 +4398,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4625,7 +4625,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4640,7 +4640,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4867,7 +4867,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4882,7 +4882,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5122,7 +5122,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5377,7 +5377,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5396,7 +5396,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5406,7 +5406,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5643,7 +5643,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5670,7 +5670,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5680,7 +5680,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5917,7 +5917,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5936,7 +5936,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5946,7 +5946,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6193,7 +6193,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6216,7 +6216,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6226,7 +6226,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6463,7 +6463,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6480,7 +6480,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6489,7 +6489,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6499,7 +6499,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6746,7 +6746,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7004,7 +7004,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7019,7 +7019,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -7447,7 +7447,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7494,7 +7494,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7504,7 +7504,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7732,7 +7732,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7747,7 +7747,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8055,7 +8055,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8082,9 +8082,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -8327,7 +8327,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8346,7 +8346,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8356,7 +8356,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8593,7 +8593,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8612,7 +8612,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8622,7 +8622,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8872,7 +8872,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8893,7 +8893,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8903,7 +8903,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9150,7 +9150,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9161,11 +9161,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -9404,7 +9404,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9423,7 +9423,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9433,7 +9433,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9753,7 +9753,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10032,7 +10032,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10051,7 +10051,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10061,7 +10061,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10324,7 +10324,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10345,7 +10345,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -10354,7 +10354,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10364,7 +10364,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_classname_tags123_api.py
index 08cf85d488bb..9e6aeb9ed9c9 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/fake_classname_tags123_api.py
@@ -250,7 +250,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/import_test_datetime_api.py
index 47876e858687..9dbb9cd39e69 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/import_test_datetime_api.py
@@ -232,7 +232,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/pet_api.py
index 5da83f4a686a..92b61d5dcbad 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/pet_api.py
@@ -255,7 +255,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -274,7 +274,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -285,7 +285,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -540,7 +540,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -553,7 +553,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -800,7 +800,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -819,7 +819,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1072,7 +1072,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1091,7 +1091,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1343,7 +1343,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1360,7 +1360,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1614,7 +1614,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1633,7 +1633,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1644,7 +1644,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1912,7 +1912,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1935,7 +1935,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1945,7 +1945,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2209,7 +2209,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2230,7 +2230,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2239,7 +2239,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2249,7 +2249,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2513,7 +2513,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2534,7 +2534,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2543,7 +2543,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2553,7 +2553,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/store_api.py
index 79116ca74ca8..5b7543c1b285 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/store_api.py
@@ -254,7 +254,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -494,7 +494,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -509,7 +509,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -759,7 +759,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -776,7 +776,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1023,7 +1023,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1040,7 +1040,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1050,7 +1050,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1060,7 +1060,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/user_api.py
index 9e4ff6771db8..387ff3f12e8d 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api/user_api.py
@@ -254,7 +254,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -273,7 +273,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -283,7 +283,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -518,7 +518,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -537,7 +537,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -547,7 +547,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -782,7 +782,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -801,7 +801,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -811,7 +811,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1051,7 +1051,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1310,7 +1310,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1327,7 +1327,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1587,7 +1587,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1610,7 +1610,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1838,7 +1838,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2105,7 +2105,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2126,7 +2126,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2136,7 +2136,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/api_client.py
index 44b7a286135f..43782b4ec85b 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ async def close(self):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -182,10 +202,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -707,7 +726,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -716,14 +739,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-httpx/petstore_api/rest.py b/samples/openapi3/client/petstore/python-httpx/petstore_api/rest.py
index e1d0094b6c56..f28745ec0fe7 100644
--- a/samples/openapi3/client/petstore/python-httpx/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-httpx/petstore_api/rest.py
@@ -119,7 +119,7 @@ async def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = httpx.Headers(headers or {})
         timeout = _request_timeout or 5 * 60
 
         if 'Content-Type' not in headers:
diff --git a/samples/openapi3/client/petstore/python-httpx/tests/test_request_headers.py b/samples/openapi3/client/petstore/python-httpx/tests/test_request_headers.py
new file mode 100644
index 000000000000..addec9e857d8
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-httpx/tests/test_request_headers.py
@@ -0,0 +1,247 @@
+"""Regression tests for request-local, case-insensitive HTTPX headers."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import importlib
+import unittest
+from typing import Any
+
+import petstore_api
+from petstore_api import rest
+
+httpx = importlib.import_module(rest.RESTResponseType.__module__.split('.')[0])
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[Any] = []
+
+        async def respond(request):
+            self.requests.append(request)
+            await asyncio.sleep(0)
+            return httpx.Response(200, json={})
+
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        self.client.rest_client.pool_manager = httpx.AsyncClient(
+            transport=httpx.MockTransport(respond)
+        )
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        await self.client.close()
+
+    def assert_single_header(self, request, name, value):
+        self.assertEqual(request.headers.get_list(name), [value])
+
+    async def test_serializers_copy_header_inputs_before_adding_defaults_and_auth(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(headers, original)
+        self.client.param_serialize(
+            'GET', '/pet', header_params=headers, query_params=[], auth_settings=['petstore_auth']
+        )
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_generated_content_type_replaces_case_variants(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'application/custom'}
+                    original = headers.copy()
+                    await self.api.add_pet(self.pet, _headers=headers)
+                    self.assert_single_header(
+                        self.requests[-1], 'content-type', 'application/json'
+                    )
+                    self.assert_single_header(self.requests[-1], 'accept', 'application/custom')
+                    self.assertEqual(headers, original)
+
+    async def test_content_type_override_and_client_default_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_and_last_inserted_case_variant_win_without_splitting_values(self):
+        self.client.set_default_header('X-Default', 'earlier-default')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'default')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-default', 'default')
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b, a')
+        self.assertIn('x-list', self.requests[-1].headers)
+        self.assertEqual(headers['X-List'], 'first')
+
+    async def test_user_agent_default_updates_case_insensitively(self):
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'custom')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_empty_request_auth_keeps_configured_authentication(self):
+        await self.api.add_pet(self.pet, _request_auth={})
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+
+    async def test_empty_cookie_does_not_add_leading_separator(self):
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params={'cookie': ''}, query_params=[],
+            auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'session=two')
+
+    async def test_explicit_header_parameter_overrides_call_headers(self):
+        headers = {'API_KEY': 'call'}
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.assertEqual(headers, {'API_KEY': 'call'})
+
+    async def test_authentication_overrides_case_variants_and_preserves_inputs(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        defaults = self.client.default_headers.copy()
+        headers = {'authorization': 'call'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.api.add_pet(self.pet, _headers=headers, _request_auth=auth)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_api_key_overrides_default_and_call_case_variants(self):
+        self.client.set_default_header('API_KEY', 'default')
+        headers = {'Api_Key': 'call'}
+        defaults = self.client.default_headers.copy()
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=headers, auth_settings=['api_key'], query_params=[]
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'api_key', 'configured-key')
+        self.assertEqual(headers, {'Api_Key': 'call'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_cookie_authentication_appends_to_selected_cookie(self):
+        self.client.cookie = 'client=1'
+        headers = {'cookie': 'call=1'}
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params=headers, query_params=[], auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'client=1; session=two')
+        self.assertEqual(headers, {'cookie': 'call=1'})
+
+    async def test_concurrent_calls_do_not_share_headers_or_authentication(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        async def call(index):
+            await self.api.add_pet(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer request-{index}',
+            })
+
+        await asyncio.gather(*(call(index) for index in range(20)))
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_public_mapping_validation_retains_combined_values(self):
+        headers = httpx.Headers([('X-List', 'a'), ('x-list', 'b')])
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b')
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        headers = httpx.Headers([
+            ('content-type', 'application/x-www-form-urlencoded'),
+            ('x-list', 'a'), ('x-list', 'b'),
+        ])
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        self.assert_single_header(
+            self.requests[-1], 'content-type', 'application/x-www-form-urlencoded'
+        )
+        self.assertEqual(self.requests[-1].content, b'field=value')
+        self.assertEqual(self.requests[-1].headers.get_list('x-list'), ['a', 'b'])
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_multipart_does_not_mutate_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        self.assertTrue(
+            self.requests[-1].headers['content-type'].startswith('multipart/form-data;')
+        )
+        self.assertEqual(len(self.requests[-1].headers.get_list('content-type')), 1)
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})
+
+    async def test_sync_calls_use_request_local_headers(self):
+        if not hasattr(self.api, 'add_pet_sync'):
+            self.skipTest('Sample does not enable supportHttpxSync')
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def call(index):
+            getattr(self.api, 'add_pet_sync')(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer sync-{index}',
+            })
+
+        def run_calls():
+            with ThreadPoolExecutor(max_workers=4) as executor:
+                list(executor.map(call, range(8)))
+
+        await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer sync-{index}' for index in range(8)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/README.md b/samples/openapi3/client/petstore/python-httpx2-sync/README.md
index 8ed8c4e6a1a7..cf31ce123a3c 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/README.md
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/README.md
@@ -329,3 +329,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/another_fake_api.py
index 3c8c561664d3..3e70d5e8c536 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/another_fake_api.py
@@ -353,7 +353,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -370,7 +370,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -379,7 +379,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -389,7 +389,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/default_api.py
index 227cb6c9bf89..0044d1e0287c 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/default_api.py
@@ -329,7 +329,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -344,7 +344,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_api.py
index 05c6c53603af..4a6f7b7183da 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_api.py
@@ -368,7 +368,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -387,7 +387,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -397,7 +397,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -733,7 +733,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1068,7 +1068,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1083,7 +1083,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1463,7 +1463,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1478,7 +1478,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1488,7 +1488,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1499,7 +1499,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1839,7 +1839,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1856,7 +1856,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1865,7 +1865,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1875,7 +1875,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2214,7 +2214,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2231,7 +2231,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2240,7 +2240,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2250,7 +2250,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2589,7 +2589,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2606,7 +2606,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2615,7 +2615,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2625,7 +2625,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2964,7 +2964,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2981,7 +2981,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2990,7 +2990,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3000,7 +3000,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3359,7 +3359,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3380,7 +3380,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -3389,7 +3389,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3399,7 +3399,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3716,7 +3716,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3731,7 +3731,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4054,7 +4054,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4069,7 +4069,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4392,7 +4392,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4407,7 +4407,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4730,7 +4730,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4745,7 +4745,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5068,7 +5068,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5083,7 +5083,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -5406,7 +5406,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5421,7 +5421,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5744,7 +5744,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5759,7 +5759,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6082,7 +6082,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6097,7 +6097,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6420,7 +6420,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6435,7 +6435,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -6758,7 +6758,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6773,7 +6773,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -7115,7 +7115,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7472,7 +7472,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7491,7 +7491,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7501,7 +7501,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7840,7 +7840,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7867,7 +7867,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7877,7 +7877,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8216,7 +8216,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8235,7 +8235,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8245,7 +8245,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8600,7 +8600,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8623,7 +8623,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8633,7 +8633,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8972,7 +8972,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8989,7 +8989,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8998,7 +8998,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9008,7 +9008,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9363,7 +9363,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9717,7 +9717,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9732,7 +9732,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -10346,7 +10346,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10393,7 +10393,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10403,7 +10403,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10727,7 +10727,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10742,7 +10742,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -11182,7 +11182,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11209,9 +11209,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -11556,7 +11556,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11575,7 +11575,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -11585,7 +11585,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -11924,7 +11924,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -11943,7 +11943,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -11953,7 +11953,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -12311,7 +12311,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -12332,7 +12332,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -12342,7 +12342,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -12697,7 +12697,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -12708,11 +12708,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -13053,7 +13053,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13072,7 +13072,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -13082,7 +13082,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -13540,7 +13540,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13921,7 +13921,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -13940,7 +13940,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -13950,7 +13950,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -14327,7 +14327,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -14348,7 +14348,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -14357,7 +14357,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -14367,7 +14367,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_classname_tags123_api.py
index 3ae5963332d5..da4d328c100f 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/fake_classname_tags123_api.py
@@ -353,7 +353,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -370,7 +370,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -379,7 +379,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -389,7 +389,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/import_test_datetime_api.py
index cf3b1e092de7..fd933e5180e0 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/import_test_datetime_api.py
@@ -329,7 +329,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -344,7 +344,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/pet_api.py
index 4228f8f36727..d304e3fba145 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/pet_api.py
@@ -358,7 +358,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -377,7 +377,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -388,7 +388,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -751,7 +751,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -764,7 +764,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -1113,7 +1113,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1132,7 +1132,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1487,7 +1487,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1506,7 +1506,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1860,7 +1860,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1877,7 +1877,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2233,7 +2233,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2252,7 +2252,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2263,7 +2263,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2645,7 +2645,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2668,7 +2668,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2678,7 +2678,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3056,7 +3056,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3077,7 +3077,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3086,7 +3086,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3096,7 +3096,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -3474,7 +3474,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3495,7 +3495,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3504,7 +3504,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -3514,7 +3514,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/store_api.py
index 3829184bb1fe..b6c56870658d 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/store_api.py
@@ -357,7 +357,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -693,7 +693,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -708,7 +708,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1060,7 +1060,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1077,7 +1077,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1426,7 +1426,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1443,7 +1443,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1453,7 +1453,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1463,7 +1463,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/user_api.py
index 85d34b6ebb62..d07e94b739fe 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api/user_api.py
@@ -357,7 +357,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -376,7 +376,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -386,7 +386,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -723,7 +723,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -742,7 +742,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -752,7 +752,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1089,7 +1089,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1108,7 +1108,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1118,7 +1118,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1460,7 +1460,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1821,7 +1821,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1838,7 +1838,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2206,7 +2206,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2229,7 +2229,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -2553,7 +2553,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2928,7 +2928,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2949,7 +2949,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2959,7 +2959,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api_client.py
index 44b7a286135f..43782b4ec85b 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ async def close(self):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -182,10 +202,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -707,7 +726,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -716,14 +739,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/rest.py b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/rest.py
index 649beee94d7b..22dc7db551e3 100644
--- a/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/petstore_api/rest.py
@@ -119,7 +119,7 @@ async def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = httpx2.Headers(headers or {})
         timeout = _request_timeout or 5 * 60
 
         if 'Content-Type' not in headers:
diff --git a/samples/openapi3/client/petstore/python-httpx2-sync/tests/test_request_headers.py b/samples/openapi3/client/petstore/python-httpx2-sync/tests/test_request_headers.py
new file mode 100644
index 000000000000..addec9e857d8
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-httpx2-sync/tests/test_request_headers.py
@@ -0,0 +1,247 @@
+"""Regression tests for request-local, case-insensitive HTTPX headers."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import importlib
+import unittest
+from typing import Any
+
+import petstore_api
+from petstore_api import rest
+
+httpx = importlib.import_module(rest.RESTResponseType.__module__.split('.')[0])
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[Any] = []
+
+        async def respond(request):
+            self.requests.append(request)
+            await asyncio.sleep(0)
+            return httpx.Response(200, json={})
+
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        self.client.rest_client.pool_manager = httpx.AsyncClient(
+            transport=httpx.MockTransport(respond)
+        )
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        await self.client.close()
+
+    def assert_single_header(self, request, name, value):
+        self.assertEqual(request.headers.get_list(name), [value])
+
+    async def test_serializers_copy_header_inputs_before_adding_defaults_and_auth(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(headers, original)
+        self.client.param_serialize(
+            'GET', '/pet', header_params=headers, query_params=[], auth_settings=['petstore_auth']
+        )
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_generated_content_type_replaces_case_variants(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'application/custom'}
+                    original = headers.copy()
+                    await self.api.add_pet(self.pet, _headers=headers)
+                    self.assert_single_header(
+                        self.requests[-1], 'content-type', 'application/json'
+                    )
+                    self.assert_single_header(self.requests[-1], 'accept', 'application/custom')
+                    self.assertEqual(headers, original)
+
+    async def test_content_type_override_and_client_default_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_and_last_inserted_case_variant_win_without_splitting_values(self):
+        self.client.set_default_header('X-Default', 'earlier-default')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'default')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-default', 'default')
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b, a')
+        self.assertIn('x-list', self.requests[-1].headers)
+        self.assertEqual(headers['X-List'], 'first')
+
+    async def test_user_agent_default_updates_case_insensitively(self):
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'custom')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_empty_request_auth_keeps_configured_authentication(self):
+        await self.api.add_pet(self.pet, _request_auth={})
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+
+    async def test_empty_cookie_does_not_add_leading_separator(self):
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params={'cookie': ''}, query_params=[],
+            auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'session=two')
+
+    async def test_explicit_header_parameter_overrides_call_headers(self):
+        headers = {'API_KEY': 'call'}
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.assertEqual(headers, {'API_KEY': 'call'})
+
+    async def test_authentication_overrides_case_variants_and_preserves_inputs(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        defaults = self.client.default_headers.copy()
+        headers = {'authorization': 'call'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.api.add_pet(self.pet, _headers=headers, _request_auth=auth)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_api_key_overrides_default_and_call_case_variants(self):
+        self.client.set_default_header('API_KEY', 'default')
+        headers = {'Api_Key': 'call'}
+        defaults = self.client.default_headers.copy()
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=headers, auth_settings=['api_key'], query_params=[]
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'api_key', 'configured-key')
+        self.assertEqual(headers, {'Api_Key': 'call'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_cookie_authentication_appends_to_selected_cookie(self):
+        self.client.cookie = 'client=1'
+        headers = {'cookie': 'call=1'}
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params=headers, query_params=[], auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'client=1; session=two')
+        self.assertEqual(headers, {'cookie': 'call=1'})
+
+    async def test_concurrent_calls_do_not_share_headers_or_authentication(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        async def call(index):
+            await self.api.add_pet(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer request-{index}',
+            })
+
+        await asyncio.gather(*(call(index) for index in range(20)))
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_public_mapping_validation_retains_combined_values(self):
+        headers = httpx.Headers([('X-List', 'a'), ('x-list', 'b')])
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b')
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        headers = httpx.Headers([
+            ('content-type', 'application/x-www-form-urlencoded'),
+            ('x-list', 'a'), ('x-list', 'b'),
+        ])
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        self.assert_single_header(
+            self.requests[-1], 'content-type', 'application/x-www-form-urlencoded'
+        )
+        self.assertEqual(self.requests[-1].content, b'field=value')
+        self.assertEqual(self.requests[-1].headers.get_list('x-list'), ['a', 'b'])
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_multipart_does_not_mutate_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        self.assertTrue(
+            self.requests[-1].headers['content-type'].startswith('multipart/form-data;')
+        )
+        self.assertEqual(len(self.requests[-1].headers.get_list('content-type')), 1)
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})
+
+    async def test_sync_calls_use_request_local_headers(self):
+        if not hasattr(self.api, 'add_pet_sync'):
+            self.skipTest('Sample does not enable supportHttpxSync')
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def call(index):
+            getattr(self.api, 'add_pet_sync')(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer sync-{index}',
+            })
+
+        def run_calls():
+            with ThreadPoolExecutor(max_workers=4) as executor:
+                list(executor.map(call, range(8)))
+
+        await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer sync-{index}' for index in range(8)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
diff --git a/samples/openapi3/client/petstore/python-httpx2/README.md b/samples/openapi3/client/petstore/python-httpx2/README.md
index 36ecb4c1a744..762afbfcdb00 100644
--- a/samples/openapi3/client/petstore/python-httpx2/README.md
+++ b/samples/openapi3/client/petstore/python-httpx2/README.md
@@ -328,3 +328,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/another_fake_api.py
index 9ceb307bbb24..968ac5d448d0 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/another_fake_api.py
@@ -250,7 +250,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/default_api.py
index dc264f69428e..5a766861f81f 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/default_api.py
@@ -232,7 +232,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_api.py
index 7fb0a185a264..8c7dc59ccaac 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_api.py
@@ -265,7 +265,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -284,7 +284,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -294,7 +294,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -528,7 +528,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -767,7 +767,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -782,7 +782,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1048,7 +1048,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1063,7 +1063,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1073,7 +1073,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1084,7 +1084,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1322,7 +1322,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1339,7 +1339,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1348,7 +1348,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1358,7 +1358,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1595,7 +1595,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1612,7 +1612,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1621,7 +1621,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1631,7 +1631,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1868,7 +1868,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1885,7 +1885,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1894,7 +1894,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1904,7 +1904,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2141,7 +2141,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2158,7 +2158,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2167,7 +2167,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2177,7 +2177,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2428,7 +2428,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2449,7 +2449,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2458,7 +2458,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2468,7 +2468,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2689,7 +2689,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2704,7 +2704,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -2931,7 +2931,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2946,7 +2946,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3173,7 +3173,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3188,7 +3188,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3415,7 +3415,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3430,7 +3430,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3657,7 +3657,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3672,7 +3672,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3899,7 +3899,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3914,7 +3914,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4141,7 +4141,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4156,7 +4156,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4383,7 +4383,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4398,7 +4398,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4625,7 +4625,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4640,7 +4640,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4867,7 +4867,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4882,7 +4882,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5122,7 +5122,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5377,7 +5377,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5396,7 +5396,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5406,7 +5406,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5643,7 +5643,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5670,7 +5670,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5680,7 +5680,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5917,7 +5917,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5936,7 +5936,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5946,7 +5946,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6193,7 +6193,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6216,7 +6216,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6226,7 +6226,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6463,7 +6463,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6480,7 +6480,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6489,7 +6489,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6499,7 +6499,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6746,7 +6746,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7004,7 +7004,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7019,7 +7019,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -7447,7 +7447,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7494,7 +7494,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7504,7 +7504,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7732,7 +7732,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7747,7 +7747,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8055,7 +8055,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8082,9 +8082,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -8327,7 +8327,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8346,7 +8346,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8356,7 +8356,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8593,7 +8593,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8612,7 +8612,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8622,7 +8622,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8872,7 +8872,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8893,7 +8893,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8903,7 +8903,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9150,7 +9150,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9161,11 +9161,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -9404,7 +9404,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9423,7 +9423,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9433,7 +9433,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9753,7 +9753,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10032,7 +10032,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10051,7 +10051,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10061,7 +10061,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10324,7 +10324,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10345,7 +10345,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -10354,7 +10354,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10364,7 +10364,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_classname_tags123_api.py
index 08cf85d488bb..9e6aeb9ed9c9 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/fake_classname_tags123_api.py
@@ -250,7 +250,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/import_test_datetime_api.py
index 47876e858687..9dbb9cd39e69 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/import_test_datetime_api.py
@@ -232,7 +232,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/pet_api.py
index 5da83f4a686a..92b61d5dcbad 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/pet_api.py
@@ -255,7 +255,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -274,7 +274,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -285,7 +285,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -540,7 +540,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -553,7 +553,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -800,7 +800,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -819,7 +819,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1072,7 +1072,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1091,7 +1091,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1343,7 +1343,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1360,7 +1360,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1614,7 +1614,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1633,7 +1633,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1644,7 +1644,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1912,7 +1912,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1935,7 +1935,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1945,7 +1945,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2209,7 +2209,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2230,7 +2230,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2239,7 +2239,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2249,7 +2249,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2513,7 +2513,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2534,7 +2534,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2543,7 +2543,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2553,7 +2553,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/store_api.py
index 79116ca74ca8..5b7543c1b285 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/store_api.py
@@ -254,7 +254,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -494,7 +494,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -509,7 +509,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -759,7 +759,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -776,7 +776,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1023,7 +1023,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1040,7 +1040,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1050,7 +1050,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1060,7 +1060,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/user_api.py
index 9e4ff6771db8..387ff3f12e8d 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api/user_api.py
@@ -254,7 +254,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -273,7 +273,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -283,7 +283,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -518,7 +518,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -537,7 +537,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -547,7 +547,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -782,7 +782,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -801,7 +801,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -811,7 +811,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1051,7 +1051,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1310,7 +1310,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1327,7 +1327,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1587,7 +1587,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1610,7 +1610,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1838,7 +1838,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2105,7 +2105,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2126,7 +2126,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2136,7 +2136,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api_client.py
index 44b7a286135f..43782b4ec85b 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ async def close(self):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -182,10 +202,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -707,7 +726,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -716,14 +739,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-httpx2/petstore_api/rest.py b/samples/openapi3/client/petstore/python-httpx2/petstore_api/rest.py
index 649beee94d7b..22dc7db551e3 100644
--- a/samples/openapi3/client/petstore/python-httpx2/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-httpx2/petstore_api/rest.py
@@ -119,7 +119,7 @@ async def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = httpx2.Headers(headers or {})
         timeout = _request_timeout or 5 * 60
 
         if 'Content-Type' not in headers:
diff --git a/samples/openapi3/client/petstore/python-httpx2/tests/test_request_headers.py b/samples/openapi3/client/petstore/python-httpx2/tests/test_request_headers.py
new file mode 100644
index 000000000000..addec9e857d8
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-httpx2/tests/test_request_headers.py
@@ -0,0 +1,247 @@
+"""Regression tests for request-local, case-insensitive HTTPX headers."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import importlib
+import unittest
+from typing import Any
+
+import petstore_api
+from petstore_api import rest
+
+httpx = importlib.import_module(rest.RESTResponseType.__module__.split('.')[0])
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[Any] = []
+
+        async def respond(request):
+            self.requests.append(request)
+            await asyncio.sleep(0)
+            return httpx.Response(200, json={})
+
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        self.client.rest_client.pool_manager = httpx.AsyncClient(
+            transport=httpx.MockTransport(respond)
+        )
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        await self.client.close()
+
+    def assert_single_header(self, request, name, value):
+        self.assertEqual(request.headers.get_list(name), [value])
+
+    async def test_serializers_copy_header_inputs_before_adding_defaults_and_auth(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(headers, original)
+        self.client.param_serialize(
+            'GET', '/pet', header_params=headers, query_params=[], auth_settings=['petstore_auth']
+        )
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_generated_content_type_replaces_case_variants(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'application/custom'}
+                    original = headers.copy()
+                    await self.api.add_pet(self.pet, _headers=headers)
+                    self.assert_single_header(
+                        self.requests[-1], 'content-type', 'application/json'
+                    )
+                    self.assert_single_header(self.requests[-1], 'accept', 'application/custom')
+                    self.assertEqual(headers, original)
+
+    async def test_content_type_override_and_client_default_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.api.add_pet(self.pet, _headers=headers, _content_type='application/custom+json')
+        self.assert_single_header(self.requests[-1], 'content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_and_last_inserted_case_variant_win_without_splitting_values(self):
+        self.client.set_default_header('X-Default', 'earlier-default')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'default')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-default', 'default')
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b, a')
+        self.assertIn('x-list', self.requests[-1].headers)
+        self.assertEqual(headers['X-List'], 'first')
+
+    async def test_user_agent_default_updates_case_insensitively(self):
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'user-agent', 'custom')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_empty_request_auth_keeps_configured_authentication(self):
+        await self.api.add_pet(self.pet, _request_auth={})
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+
+    async def test_empty_cookie_does_not_add_leading_separator(self):
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params={'cookie': ''}, query_params=[],
+            auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'session=two')
+
+    async def test_explicit_header_parameter_overrides_call_headers(self):
+        headers = {'API_KEY': 'call'}
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.assertEqual(headers, {'API_KEY': 'call'})
+
+    async def test_authentication_overrides_case_variants_and_preserves_inputs(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        defaults = self.client.default_headers.copy()
+        headers = {'authorization': 'call'}
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.api.add_pet(self.pet, _headers=headers, _request_auth=auth)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_api_key_overrides_default_and_call_case_variants(self):
+        self.client.set_default_header('API_KEY', 'default')
+        headers = {'Api_Key': 'call'}
+        defaults = self.client.default_headers.copy()
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=headers, auth_settings=['api_key'], query_params=[]
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'api_key', 'configured-key')
+        self.assertEqual(headers, {'Api_Key': 'call'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_cookie_authentication_appends_to_selected_cookie(self):
+        self.client.cookie = 'client=1'
+        headers = {'cookie': 'call=1'}
+        params = self.client.param_serialize(
+            'GET', '/cookie', header_params=headers, query_params=[], auth_settings=['cookie'],
+            _request_auth={'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two'},
+        )
+        await self.client.call_api(*params)
+        self.assert_single_header(self.requests[-1], 'cookie', 'client=1; session=two')
+        self.assertEqual(headers, {'cookie': 'call=1'})
+
+    async def test_concurrent_calls_do_not_share_headers_or_authentication(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        async def call(index):
+            await self.api.add_pet(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer request-{index}',
+            })
+
+        await asyncio.gather(*(call(index) for index in range(20)))
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        await self.api.add_pet(self.pet)
+        self.assert_single_header(self.requests[-1], 'authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_public_mapping_validation_retains_combined_values(self):
+        headers = httpx.Headers([('X-List', 'a'), ('x-list', 'b')])
+        await self.api.add_pet(self.pet, _headers=headers)
+        self.assert_single_header(self.requests[-1], 'x-list', 'a, b')
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        headers = httpx.Headers([
+            ('content-type', 'application/x-www-form-urlencoded'),
+            ('x-list', 'a'), ('x-list', 'b'),
+        ])
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        self.assert_single_header(
+            self.requests[-1], 'content-type', 'application/x-www-form-urlencoded'
+        )
+        self.assertEqual(self.requests[-1].content, b'field=value')
+        self.assertEqual(self.requests[-1].headers.get_list('x-list'), ['a', 'b'])
+        self.assertEqual(headers.get_list('x-list'), ['a', 'b'])
+
+    async def test_transport_multipart_does_not_mutate_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        await self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        self.assertTrue(
+            self.requests[-1].headers['content-type'].startswith('multipart/form-data;')
+        )
+        self.assertEqual(len(self.requests[-1].headers.get_list('content-type')), 1)
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})
+
+    async def test_sync_calls_use_request_local_headers(self):
+        if not hasattr(self.api, 'add_pet_sync'):
+            self.skipTest('Sample does not enable supportHttpxSync')
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def call(index):
+            getattr(self.api, 'add_pet_sync')(self.pet, _headers=headers, _request_auth={
+                'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                'value': f'Bearer sync-{index}',
+            })
+
+        def run_calls():
+            with ThreadPoolExecutor(max_workers=4) as executor:
+                list(executor.map(call, range(8)))
+
+        await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request.headers['authorization'] for request in self.requests],
+            [f'Bearer sync-{index}' for index in range(8)],
+        )
+        for request in self.requests:
+            self.assert_single_header(request, 'content-type', 'application/json')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
diff --git a/samples/openapi3/client/petstore/python-lazyImports/README.md b/samples/openapi3/client/petstore/python-lazyImports/README.md
index eff9ffcd975e..d907bad2411f 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/README.md
+++ b/samples/openapi3/client/petstore/python-lazyImports/README.md
@@ -349,3 +349,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/another_fake_api.py
index 128465621cbc..cf0ad78f8b88 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/another_fake_api.py
@@ -270,7 +270,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -287,7 +287,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -296,7 +296,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -306,7 +306,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/default_api.py
index 25a038d2c23d..1d78bf1f8fe5 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/default_api.py
@@ -252,7 +252,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_api.py
index b2e1cd1be926..b7d2c36217b7 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_api.py
@@ -285,7 +285,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -304,7 +304,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -314,7 +314,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -548,7 +548,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -787,7 +787,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -802,7 +802,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1068,7 +1068,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1083,7 +1083,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1093,7 +1093,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1104,7 +1104,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1342,7 +1342,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1359,7 +1359,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1368,7 +1368,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1378,7 +1378,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1615,7 +1615,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1632,7 +1632,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1641,7 +1641,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1651,7 +1651,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1888,7 +1888,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1905,7 +1905,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1914,7 +1914,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1924,7 +1924,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2161,7 +2161,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2178,7 +2178,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2187,7 +2187,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2197,7 +2197,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2448,7 +2448,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2469,7 +2469,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2478,7 +2478,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2488,7 +2488,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2709,7 +2709,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2724,7 +2724,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -2951,7 +2951,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2966,7 +2966,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3193,7 +3193,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3208,7 +3208,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3435,7 +3435,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3450,7 +3450,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3677,7 +3677,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3692,7 +3692,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3919,7 +3919,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3934,7 +3934,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4161,7 +4161,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4176,7 +4176,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4403,7 +4403,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4418,7 +4418,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4645,7 +4645,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4660,7 +4660,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4887,7 +4887,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4902,7 +4902,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5142,7 +5142,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5397,7 +5397,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5416,7 +5416,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5426,7 +5426,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5663,7 +5663,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5690,7 +5690,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5700,7 +5700,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5937,7 +5937,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5956,7 +5956,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5966,7 +5966,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6213,7 +6213,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6236,7 +6236,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6246,7 +6246,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6483,7 +6483,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6500,7 +6500,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6509,7 +6509,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6519,7 +6519,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6766,7 +6766,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7024,7 +7024,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7039,7 +7039,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -7467,7 +7467,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7514,7 +7514,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7524,7 +7524,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7752,7 +7752,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7767,7 +7767,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8075,7 +8075,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8102,9 +8102,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -8347,7 +8347,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8366,7 +8366,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8376,7 +8376,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8613,7 +8613,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8632,7 +8632,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8642,7 +8642,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8892,7 +8892,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8913,7 +8913,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8923,7 +8923,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9170,7 +9170,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9181,11 +9181,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -9424,7 +9424,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9443,7 +9443,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9453,7 +9453,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9773,7 +9773,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10052,7 +10052,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10071,7 +10071,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10081,7 +10081,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10344,7 +10344,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10365,7 +10365,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -10374,7 +10374,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10384,7 +10384,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_classname_tags123_api.py
index d9f5f5a570be..8faa294b368f 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/fake_classname_tags123_api.py
@@ -270,7 +270,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -287,7 +287,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -296,7 +296,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -306,7 +306,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/import_test_datetime_api.py
index 1045da8de511..1f7a61b06d09 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/import_test_datetime_api.py
@@ -252,7 +252,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/pet_api.py
index 2dfc90005d39..1ea2684a449d 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/pet_api.py
@@ -275,7 +275,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -294,7 +294,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -305,7 +305,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -560,7 +560,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -573,7 +573,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -820,7 +820,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -839,7 +839,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1092,7 +1092,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1111,7 +1111,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1363,7 +1363,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1380,7 +1380,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1634,7 +1634,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1653,7 +1653,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1664,7 +1664,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1932,7 +1932,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1955,7 +1955,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1965,7 +1965,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2229,7 +2229,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2250,7 +2250,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2259,7 +2259,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2269,7 +2269,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2533,7 +2533,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2554,7 +2554,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2563,7 +2563,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2573,7 +2573,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/store_api.py
index 6e3df4213355..c579ee54e748 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/store_api.py
@@ -274,7 +274,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -514,7 +514,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -529,7 +529,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -779,7 +779,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -796,7 +796,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1043,7 +1043,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1060,7 +1060,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1070,7 +1070,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1080,7 +1080,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/user_api.py
index 59ce49f2f797..321881637df4 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api/user_api.py
@@ -274,7 +274,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -293,7 +293,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -303,7 +303,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -538,7 +538,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -557,7 +557,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -567,7 +567,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -802,7 +802,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -821,7 +821,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -831,7 +831,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1071,7 +1071,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1330,7 +1330,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1347,7 +1347,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1607,7 +1607,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1630,7 +1630,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1858,7 +1858,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2125,7 +2125,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2146,7 +2146,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2156,7 +2156,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api_client.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api_client.py
index eeb694ac158b..e94f64789c50 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -106,16 +106,36 @@ def __exit__(self, exc_type, exc_value, traceback):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -179,10 +199,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -704,7 +723,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -713,14 +736,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/rest.py b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/rest.py
index 8affe12ad0a2..abd28ce48ff1 100644
--- a/samples/openapi3/client/petstore/python-lazyImports/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python-lazyImports/petstore_api/rest.py
@@ -219,7 +219,7 @@ def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/samples/openapi3/client/petstore/python/README.md b/samples/openapi3/client/petstore/python/README.md
index a7585af3cc78..f50f803b5806 100755
--- a/samples/openapi3/client/petstore/python/README.md
+++ b/samples/openapi3/client/petstore/python/README.md
@@ -326,3 +326,33 @@ Authentication schemes defined for the API:
 
 
 
+
+## Request headers
+
+Header names are compared case-insensitively in all Python client libraries.
+Request headers are copied; supplying `_headers` does not change the caller's dict
+or the client's shared defaults. The public `_headers` argument retains its dict
+validation and existing value serialization. Pydantic can coerce mappings such as HTTPX `Headers` to a dict,
+combining repeated fields into a single value; `_headers` does not provide a
+repeated-field or list-of-pairs API.
+
+Precedence, from lowest to highest:
+
+1. `_headers` (within one dict, the last inserted case variant wins).
+2. Explicit OpenAPI header parameters and generated `Content-Type` (or `_content_type`).
+   Generated `Accept` is added only when neither `_headers` nor explicit header
+   parameters contain an `Accept` case variant.
+3. `ApiClient.default_headers` (including headers set with `set_default_header`).
+4. The client's `cookie` setting, for `Cookie`.
+5. Authentication headers. A non-empty `_request_auth` replaces configured
+   authentication for that call; `{}` retains configured authentication. Cookie
+   authentication appends to the selected `Cookie` value.
+
+This retains the existing precedence for identically spelled names and extends it
+to case variants. To override a generated content type, use `_content_type`;
+client defaults still take precedence over it. The winning header's spelling and
+value are retained for replacements. Cookie authentication uses the canonical
+`Cookie` spelling and appends its value. Values containing legitimate
+comma-separated lists are not split or deduplicated. This does not remove
+duplicate values inside a header or change the handling of repeated fields
+supplied directly to the transport.
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/another_fake_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/another_fake_api.py
index 56f2ab2cd1cd..d75e24eca3a5 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/another_fake_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/another_fake_api.py
@@ -250,7 +250,7 @@ def _call_123_test_special_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _call_123_test_special_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _call_123_test_special_tags_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _call_123_test_special_tags_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/default_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/default_api.py
index ce4db6893d28..6abe92c60307 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/default_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/default_api.py
@@ -232,7 +232,7 @@ def _foo_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _foo_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/fake_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/fake_api.py
index fca5d8161a3d..020ebbfa43ae 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/fake_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/fake_api.py
@@ -265,7 +265,7 @@ def _fake_any_type_request_body_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -284,7 +284,7 @@ def _fake_any_type_request_body_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -294,7 +294,7 @@ def _fake_any_type_request_body_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -528,7 +528,7 @@ def _fake_enum_ref_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -767,7 +767,7 @@ def _fake_health_get_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -782,7 +782,7 @@ def _fake_health_get_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -1048,7 +1048,7 @@ def _fake_http_signature_test_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1063,7 +1063,7 @@ def _fake_http_signature_test_serialize(
             
         # process the header parameters
         if header_1 is not None:
-            _header_params['header_1'] = header_1
+            self.api_client._set_header(_header_params, 'header_1', header_1)
         # process the form parameters
         # process the body parameter
         if pet is not None:
@@ -1073,7 +1073,7 @@ def _fake_http_signature_test_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1084,7 +1084,7 @@ def _fake_http_signature_test_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1322,7 +1322,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1339,7 +1339,7 @@ def _fake_outer_boolean_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1348,7 +1348,7 @@ def _fake_outer_boolean_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1358,7 +1358,7 @@ def _fake_outer_boolean_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1595,7 +1595,7 @@ def _fake_outer_composite_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1612,7 +1612,7 @@ def _fake_outer_composite_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1621,7 +1621,7 @@ def _fake_outer_composite_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1631,7 +1631,7 @@ def _fake_outer_composite_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1868,7 +1868,7 @@ def _fake_outer_number_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1885,7 +1885,7 @@ def _fake_outer_number_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -1894,7 +1894,7 @@ def _fake_outer_number_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1904,7 +1904,7 @@ def _fake_outer_number_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2141,7 +2141,7 @@ def _fake_outer_string_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2158,7 +2158,7 @@ def _fake_outer_string_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2167,7 +2167,7 @@ def _fake_outer_string_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2177,7 +2177,7 @@ def _fake_outer_string_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2428,7 +2428,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2449,7 +2449,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     '*/*'
@@ -2458,7 +2458,7 @@ def _fake_property_enum_integer_serialize_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2468,7 +2468,7 @@ def _fake_property_enum_integer_serialize_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2689,7 +2689,7 @@ def _fake_ref_enum_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2704,7 +2704,7 @@ def _fake_ref_enum_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -2931,7 +2931,7 @@ def _fake_return_boolean_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2946,7 +2946,7 @@ def _fake_return_boolean_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3173,7 +3173,7 @@ def _fake_return_byte_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3188,7 +3188,7 @@ def _fake_return_byte_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3415,7 +3415,7 @@ def _fake_return_enum_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3430,7 +3430,7 @@ def _fake_return_enum_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -3657,7 +3657,7 @@ def _fake_return_enum_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3672,7 +3672,7 @@ def _fake_return_enum_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -3899,7 +3899,7 @@ def _fake_return_float_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -3914,7 +3914,7 @@ def _fake_return_float_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4141,7 +4141,7 @@ def _fake_return_int_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4156,7 +4156,7 @@ def _fake_return_int_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4383,7 +4383,7 @@ def _fake_return_list_of_objects_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4398,7 +4398,7 @@ def _fake_return_list_of_objects_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -4625,7 +4625,7 @@ def _fake_return_str_like_json_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4640,7 +4640,7 @@ def _fake_return_str_like_json_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'plain/text'
@@ -4867,7 +4867,7 @@ def _fake_return_string_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -4882,7 +4882,7 @@ def _fake_return_string_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -5122,7 +5122,7 @@ def _fake_uuid_example_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5377,7 +5377,7 @@ def _test_additional_properties_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5396,7 +5396,7 @@ def _test_additional_properties_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5406,7 +5406,7 @@ def _test_additional_properties_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5643,7 +5643,7 @@ def _test_body_with_binary_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5670,7 +5670,7 @@ def _test_body_with_binary_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5680,7 +5680,7 @@ def _test_body_with_binary_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -5917,7 +5917,7 @@ def _test_body_with_file_schema_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -5936,7 +5936,7 @@ def _test_body_with_file_schema_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -5946,7 +5946,7 @@ def _test_body_with_file_schema_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6193,7 +6193,7 @@ def _test_body_with_query_params_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6216,7 +6216,7 @@ def _test_body_with_query_params_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6226,7 +6226,7 @@ def _test_body_with_query_params_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6463,7 +6463,7 @@ def _test_client_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -6480,7 +6480,7 @@ def _test_client_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -6489,7 +6489,7 @@ def _test_client_model_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -6499,7 +6499,7 @@ def _test_client_model_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -6746,7 +6746,7 @@ def _test_date_time_query_parameter_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7004,7 +7004,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7019,7 +7019,7 @@ def _test_empty_and_non_empty_responses_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'text/plain'
@@ -7447,7 +7447,7 @@ def _test_endpoint_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7494,7 +7494,7 @@ def _test_endpoint_parameters_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -7504,7 +7504,7 @@ def _test_endpoint_parameters_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -7732,7 +7732,7 @@ def _test_error_responses_with_model_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -7747,7 +7747,7 @@ def _test_error_responses_with_model_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -8055,7 +8055,7 @@ def _test_group_parameters_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8082,9 +8082,9 @@ def _test_group_parameters_serialize(
             
         # process the header parameters
         if required_boolean_group is not None:
-            _header_params['required_boolean_group'] = required_boolean_group
+            self.api_client._set_header(_header_params, 'required_boolean_group', required_boolean_group)
         if boolean_group is not None:
-            _header_params['boolean_group'] = boolean_group
+            self.api_client._set_header(_header_params, 'boolean_group', boolean_group)
         # process the form parameters
         # process the body parameter
 
@@ -8327,7 +8327,7 @@ def _test_inline_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8346,7 +8346,7 @@ def _test_inline_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8356,7 +8356,7 @@ def _test_inline_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8593,7 +8593,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8612,7 +8612,7 @@ def _test_inline_freeform_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8622,7 +8622,7 @@ def _test_inline_freeform_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -8872,7 +8872,7 @@ def _test_json_form_data_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -8893,7 +8893,7 @@ def _test_json_form_data_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -8903,7 +8903,7 @@ def _test_json_form_data_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9150,7 +9150,7 @@ def _test_json_header_content_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9161,11 +9161,11 @@ def _test_json_header_content_serialize(
         # process the query parameters
         # process the header parameters
         if x_json_arg is not None:
-            _header_params['X-Json-Arg'] = json.dumps(
+            self.api_client._set_header(_header_params, 'X-Json-Arg', json.dumps(
                 self.api_client.sanitize_for_serialization(x_json_arg)
-            )
+            ))
         if x_plain_arg is not None:
-            _header_params['X-Plain-Arg'] = x_plain_arg
+            self.api_client._set_header(_header_params, 'X-Plain-Arg', x_plain_arg)
         # process the form parameters
         # process the body parameter
 
@@ -9404,7 +9404,7 @@ def _test_object_for_multipart_requests_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -9423,7 +9423,7 @@ def _test_object_for_multipart_requests_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -9433,7 +9433,7 @@ def _test_object_for_multipart_requests_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -9753,7 +9753,7 @@ def _test_query_parameter_collection_format_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10032,7 +10032,7 @@ def _test_string_map_reference_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10051,7 +10051,7 @@ def _test_string_map_reference_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10061,7 +10061,7 @@ def _test_string_map_reference_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -10324,7 +10324,7 @@ def _upload_file_with_additional_properties_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -10345,7 +10345,7 @@ def _upload_file_with_additional_properties_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -10354,7 +10354,7 @@ def _upload_file_with_additional_properties_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -10364,7 +10364,7 @@ def _upload_file_with_additional_properties_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/fake_classname_tags123_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/fake_classname_tags123_api.py
index e373462e5f5b..bdcd298ac5db 100644
--- a/samples/openapi3/client/petstore/python/petstore_api/api/fake_classname_tags123_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/fake_classname_tags123_api.py
@@ -250,7 +250,7 @@ def _test_classname_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -267,7 +267,7 @@ def _test_classname_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -276,7 +276,7 @@ def _test_classname_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -286,7 +286,7 @@ def _test_classname_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/import_test_datetime_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/import_test_datetime_api.py
index ae01fa137fa1..80426d1910ad 100644
--- a/samples/openapi3/client/petstore/python/petstore_api/api/import_test_datetime_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/import_test_datetime_api.py
@@ -232,7 +232,7 @@ def _import_test_return_datetime_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -247,7 +247,7 @@ def _import_test_return_datetime_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/pet_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/pet_api.py
index 7d4cf604eb15..d34bd5c3c4e7 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/pet_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/pet_api.py
@@ -255,7 +255,7 @@ def _add_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -274,7 +274,7 @@ def _add_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -285,7 +285,7 @@ def _add_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -540,7 +540,7 @@ def _delete_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -553,7 +553,7 @@ def _delete_pet_serialize(
         # process the query parameters
         # process the header parameters
         if api_key is not None:
-            _header_params['api_key'] = api_key
+            self.api_client._set_header(_header_params, 'api_key', api_key)
         # process the form parameters
         # process the body parameter
 
@@ -800,7 +800,7 @@ def _find_pets_by_status_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -819,7 +819,7 @@ def _find_pets_by_status_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1072,7 +1072,7 @@ def _find_pets_by_tags_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1091,7 +1091,7 @@ def _find_pets_by_tags_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1343,7 +1343,7 @@ def _get_pet_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1360,7 +1360,7 @@ def _get_pet_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1614,7 +1614,7 @@ def _update_pet_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1633,7 +1633,7 @@ def _update_pet_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1644,7 +1644,7 @@ def _update_pet_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1912,7 +1912,7 @@ def _update_pet_with_form_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1935,7 +1935,7 @@ def _update_pet_with_form_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1945,7 +1945,7 @@ def _update_pet_with_form_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2209,7 +2209,7 @@ def _upload_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2230,7 +2230,7 @@ def _upload_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2239,7 +2239,7 @@ def _upload_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2249,7 +2249,7 @@ def _upload_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -2513,7 +2513,7 @@ def _upload_file_with_required_file_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2534,7 +2534,7 @@ def _upload_file_with_required_file_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -2543,7 +2543,7 @@ def _upload_file_with_required_file_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2553,7 +2553,7 @@ def _upload_file_with_required_file_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/store_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/store_api.py
index 73471217df00..f323e37fcdd5 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/store_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/store_api.py
@@ -254,7 +254,7 @@ def _delete_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -494,7 +494,7 @@ def _get_inventory_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -509,7 +509,7 @@ def _get_inventory_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/json'
@@ -759,7 +759,7 @@ def _get_order_by_id_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -776,7 +776,7 @@ def _get_order_by_id_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1023,7 +1023,7 @@ def _place_order_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1040,7 +1040,7 @@ def _place_order_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1050,7 +1050,7 @@ def _place_order_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -1060,7 +1060,7 @@ def _place_order_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api/user_api.py b/samples/openapi3/client/petstore/python/petstore_api/api/user_api.py
index be61d536e31c..28e2ca3c42e1 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api/user_api.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api/user_api.py
@@ -254,7 +254,7 @@ def _create_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -273,7 +273,7 @@ def _create_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -283,7 +283,7 @@ def _create_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -518,7 +518,7 @@ def _create_users_with_array_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -537,7 +537,7 @@ def _create_users_with_array_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -547,7 +547,7 @@ def _create_users_with_array_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -782,7 +782,7 @@ def _create_users_with_list_input_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -801,7 +801,7 @@ def _create_users_with_list_input_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -811,7 +811,7 @@ def _create_users_with_list_input_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
@@ -1051,7 +1051,7 @@ def _delete_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1310,7 +1310,7 @@ def _get_user_by_name_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1327,7 +1327,7 @@ def _get_user_by_name_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1587,7 +1587,7 @@ def _login_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -1610,7 +1610,7 @@ def _login_user_serialize(
 
 
         # set the HTTP header `Accept`
-        if 'Accept' not in _header_params:
+        if not any(key.lower() == 'accept' for key in _header_params):
             _header_params['Accept'] = self.api_client.select_header_accept(
                 [
                     'application/xml', 
@@ -1838,7 +1838,7 @@ def _logout_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2105,7 +2105,7 @@ def _update_user_serialize(
 
         _path_params: Dict[str, str] = {}
         _query_params: List[Tuple[str, str]] = []
-        _header_params: Dict[str, Optional[str]] = _headers or {}
+        _header_params: Dict[str, Any] = self.api_client._merge_headers(_headers)
         _form_params: List[Tuple[str, str]] = []
         _files: Dict[
             str, Union[str, bytes, List[str], List[bytes], List[Tuple[str, bytes]]]
@@ -2126,7 +2126,7 @@ def _update_user_serialize(
 
         # set the HTTP header `Content-Type`
         if _content_type:
-            _header_params['Content-Type'] = _content_type
+            self.api_client._set_header(_header_params, 'Content-Type', _content_type)
         else:
             _default_content_type = (
                 self.api_client.select_header_content_type(
@@ -2136,7 +2136,7 @@ def _update_user_serialize(
                 )
             )
             if _default_content_type is not None:
-                _header_params['Content-Type'] = _default_content_type
+                self.api_client._set_header(_header_params, 'Content-Type', _default_content_type)
 
         # authentication setting
         _auth_settings: List[str] = [
diff --git a/samples/openapi3/client/petstore/python/petstore_api/api_client.py b/samples/openapi3/client/petstore/python/petstore_api/api_client.py
index c98eb486e651..5b6101731ba0 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/api_client.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/api_client.py
@@ -23,7 +23,7 @@
 import uuid
 
 from urllib.parse import quote
-from typing import Tuple, Optional, List, Dict, Union
+from typing import Any, Tuple, Optional, List, Dict, Union
 from pydantic import SecretStr
 
 from petstore_api.configuration import Configuration
@@ -103,16 +103,36 @@ def __exit__(self, exc_type, exc_value, traceback):
     @property
     def user_agent(self):
         """User agent for this API client"""
-        return self.default_headers['User-Agent']
+        for name, value in reversed(self.default_headers.items()):
+            if name.lower() == 'user-agent':
+                return value
+        raise KeyError('User-Agent')
 
     @user_agent.setter
     def user_agent(self, value):
-        self.default_headers['User-Agent'] = value
+        self._set_header(self.default_headers, 'User-Agent', value)
 
     def set_default_header(self, header_name, header_value):
-        self.default_headers[header_name] = header_value
+        self._set_header(self.default_headers, header_name, header_value)
 
 
+    @staticmethod
+    def _set_header(headers: Dict[str, Any], name: str, value: Any) -> None:
+        """Replace a header case-insensitively, retaining the winning spelling."""
+        for key in list(headers):
+            if key.lower() == name.lower():
+                del headers[key]
+        headers[name] = value
+
+    @classmethod
+    def _merge_headers(cls, *sources: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+        """Copy headers; later sources and later entries in each dict win."""
+        headers: Dict[str, Any] = {}
+        for source in sources:
+            for name, value in (source or {}).items():
+                cls._set_header(headers, name, value)
+        return headers
+
     _default = None
 
     @classmethod
@@ -179,10 +199,9 @@ def param_serialize(
         config = self.configuration
 
         # header parameters
-        header_params = header_params or {}
-        header_params.update(self.default_headers)
+        header_params = self._merge_headers(header_params, self.default_headers)
         if self.cookie:
-            header_params['Cookie'] = self.cookie
+            self._set_header(header_params, 'Cookie', self.cookie)
         if header_params:
             header_params = self.sanitize_for_serialization(header_params)
             header_params = dict(
@@ -704,7 +723,11 @@ def _apply_auth_params(
         :param auth_setting: auth settings for the endpoint
         """
         if auth_setting['in'] == 'cookie':
-            if not 'Cookie' in headers:
+            for key in list(headers):
+                if key.lower() == 'cookie':
+                    self._set_header(headers, 'Cookie', headers[key])
+                    break
+            if not headers.get('Cookie'):
                 headers['Cookie'] = ""
             else:
                 headers['Cookie'] += "; "
@@ -713,14 +736,15 @@ def _apply_auth_params(
             headers['Cookie'] += f"{auth_setting['key']}={cookie_value}"
         elif auth_setting['in'] == 'header':
             if auth_setting['type'] != 'http-signature':
-                headers[auth_setting['key']] = auth_setting['value']
+                self._set_header(headers, auth_setting['key'], auth_setting['value'])
             else:
                 # The HTTP signature scheme requires multiple HTTP headers
                 # that are calculated dynamically.
                 signing_info = self.configuration.signing_info
                 auth_headers = signing_info.get_http_signature_headers(
                 resource_path, method, headers, body, queries)
-                headers.update(auth_headers)
+                for name, value in auth_headers.items():
+                    self._set_header(headers, name, value)
         elif auth_setting['in'] == 'query':
             queries.append((auth_setting['key'], auth_setting['value']))
         else:
diff --git a/samples/openapi3/client/petstore/python/petstore_api/rest.py b/samples/openapi3/client/petstore/python/petstore_api/rest.py
index a265bee3515c..b50e20bcbaae 100755
--- a/samples/openapi3/client/petstore/python/petstore_api/rest.py
+++ b/samples/openapi3/client/petstore/python/petstore_api/rest.py
@@ -216,7 +216,7 @@ def request(
             )
 
         post_params = post_params or {}
-        headers = headers or {}
+        headers = urllib3.HTTPHeaderDict(headers or {})
 
         timeout = None
         if _request_timeout:
diff --git a/samples/openapi3/client/petstore/python/tests/test_request_headers.py b/samples/openapi3/client/petstore/python/tests/test_request_headers.py
new file mode 100644
index 000000000000..275c483d0e2b
--- /dev/null
+++ b/samples/openapi3/client/petstore/python/tests/test_request_headers.py
@@ -0,0 +1,227 @@
+"""Request-header behavior for the urllib3 client."""
+
+import asyncio
+from concurrent.futures import ThreadPoolExecutor
+import inspect
+import unittest
+from typing import Any
+from unittest.mock import AsyncMock, Mock
+
+import petstore_api
+
+ASYNC = inspect.iscoroutinefunction(petstore_api.ApiClient.call_api)
+
+
+class TestRequestHeaders(unittest.IsolatedAsyncioTestCase):
+    async def asyncSetUp(self):
+        self.requests: list[dict[str, Any]] = []
+        self.client = petstore_api.ApiClient(petstore_api.Configuration(
+            host='https://example.test', access_token='configured-token',
+            api_key={'api_key': 'configured-key'},
+        ))
+        response = Mock(status=200, reason='OK', data=b'{}', headers={})
+        response.read = AsyncMock(return_value=b'{}')
+
+        def respond(*args, **kwargs):
+            self.requests.append({**kwargs, 'headers': kwargs['headers'].copy()})
+            return response
+
+        async def respond_async(*args, **kwargs):
+            await asyncio.sleep(0)
+            return respond(*args, **kwargs)
+
+        self.pool: Any = AsyncMock() if ASYNC else Mock()
+        self.pool.request.side_effect = respond_async if ASYNC else respond
+        self.client.rest_client.pool_manager = self.pool
+        self.api = petstore_api.PetApi(self.client)
+        self.pet = petstore_api.Pet(name='test', photoUrls=[])
+
+    async def asyncTearDown(self):
+        if ASYNC:
+            await getattr(self.client, 'close')()
+        else:
+            self.pool.clear()
+
+    async def call(self, **kwargs):
+        result = self.api.add_pet(self.pet, **kwargs)
+        if inspect.isawaitable(result):
+            await result
+
+    def values(self, headers, name):
+        if isinstance(headers, dict):
+            return [value for key, value in headers.items() if key.lower() == name.lower()]
+        return headers.getall(name) if ASYNC else headers.getlist(name)
+
+    def assert_header(self, name, value):
+        self.assertEqual(self.values(self.requests[-1]['headers'], name), [value])
+
+    async def test_generated_content_type_and_accept_are_case_insensitive(self):
+        for spelling in ('Content-Type', 'content-type', 'CONTENT-TYPE'):
+            for value in ('application/json', 'text/plain'):
+                with self.subTest(spelling=spelling, value=value):
+                    headers = {spelling: value, 'accept': 'custom'}
+                    await self.call(_headers=headers)
+                    self.assert_header('content-type', 'application/json')
+                    self.assert_header('accept', 'custom')
+                    self.assertEqual(headers, {spelling: value, 'accept': 'custom'})
+
+    async def test_user_agent_matches_last_direct_default_header_write(self):
+        self.client.default_headers['user-agent'] = 'custom'
+        self.client.default_headers['USER-AGENT'] = 'latest'
+        defaults = self.client.default_headers.copy()
+        self.assertEqual(self.client.user_agent, 'latest')
+        await self.call()
+        self.assert_header('user-agent', 'latest')
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_content_type_override_and_defaults_retain_precedence(self):
+        headers = {'content-type': 'text/plain'}
+        await self.call(_headers=headers, _content_type='application/custom+json')
+        self.assert_header('content-type', 'application/custom+json')
+        self.client.set_default_header('CONTENT-TYPE', 'application/default+json')
+        defaults = self.client.default_headers.copy()
+        await self.call(_headers=headers, _content_type='application/custom+json')
+        self.assert_header('content-type', 'application/default+json')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.assertEqual(headers, {'content-type': 'text/plain'})
+
+    async def test_defaults_last_write_and_user_agent_property(self):
+        self.client.set_default_header('X-Default', 'old')
+        self.client.set_default_header('x-default', 'middle')
+        self.client.set_default_header('X-Default', 'new')
+        self.client.set_default_header('user-agent', 'custom')
+        self.assertEqual(self.client.user_agent, 'custom')
+        headers = {'x-default': 'call', 'X-List': 'first', 'x-list': 'a, b, a'}
+        await self.call(_headers=headers)
+        self.assert_header('x-default', 'new')
+        self.assert_header('user-agent', 'custom')
+        self.assert_header('x-list', 'a, b, a')
+        self.assertEqual(headers['X-List'], 'first')
+        self.client.user_agent = 'reset'
+        self.assertEqual(self.client.user_agent, 'reset')
+        self.assertEqual(
+            len([key for key in self.client.default_headers if key.lower() == 'user-agent']), 1
+        )
+
+    async def test_serializers_preserve_inputs_and_explicit_header_wins(self):
+        headers = {'content-type': 'text/plain', 'API_KEY': 'call'}
+        original = headers.copy()
+        defaults = self.client.default_headers.copy()
+        params = self.api._delete_pet_serialize(
+            pet_id=1, api_key='explicit', _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.assertEqual(params[2]['api_key'], 'explicit')
+        self.api._add_pet_serialize(
+            pet=self.pet, _headers=headers, _request_auth=None,
+            _content_type=None, _host_index=0,
+        )
+        self.client.param_serialize('GET', '/pet', header_params=headers, query_params=[])
+        self.assertEqual(headers, original)
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_authentication_and_empty_override(self):
+        self.client.set_default_header('AUTHORIZATION', 'default')
+        headers = {'authorization': 'call'}
+        defaults = self.client.default_headers.copy()
+        await self.call(_headers=headers, _request_auth={})
+        self.assert_header('authorization', 'Bearer configured-token')
+        auth = {'in': 'header', 'key': 'Authorization', 'type': 'oauth2', 'value': 'Bearer call'}
+        await self.call(_headers=headers, _request_auth=auth)
+        self.assert_header('authorization', 'Bearer call')
+        self.assertEqual(headers, {'authorization': 'call'})
+        self.assertEqual(auth['value'], 'Bearer call')
+        self.assertEqual(self.client.default_headers, defaults)
+        self.client.set_default_header('API_KEY', 'default')
+        key_headers = {'Api_Key': 'call'}
+        params = self.client.param_serialize(
+            'GET', '/key', header_params=key_headers, auth_settings=['api_key'], query_params=[]
+        )
+        result = self.client.call_api(*params)
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('api_key', 'configured-key')
+        self.assertEqual(key_headers, {'Api_Key': 'call'})
+
+    async def test_cookie_authentication_appends_without_empty_separator(self):
+        for value, expected in (('', 'session=two'), ('client=1', 'client=1; session=two')):
+            headers = {'cookie': value}
+            params = self.client.param_serialize(
+                'GET', '/cookie', header_params=headers, query_params=[],
+                auth_settings=['cookie'],
+                _request_auth={
+                    'in': 'cookie', 'key': 'session', 'type': 'api_key', 'value': 'two',
+                },
+            )
+            result = self.client.call_api(*params)
+            if inspect.isawaitable(result):
+                await result
+            self.assert_header('cookie', expected)
+            self.assertEqual(headers, {'cookie': value})
+
+    async def test_concurrent_calls_keep_inputs_defaults_and_credentials_isolated(self):
+        headers = {'content-type': 'application/json', 'authorization': 'caller'}
+        defaults = self.client.default_headers.copy()
+
+        def auth(index):
+            return {'in': 'header', 'key': 'Authorization', 'type': 'oauth2',
+                    'value': f'Bearer request-{index}'}
+
+        if ASYNC:
+            await asyncio.gather(*(
+                self.call(_headers=headers, _request_auth=auth(index)) for index in range(20)
+            ))
+        else:
+            def run_calls():
+                with ThreadPoolExecutor(max_workers=4) as executor:
+                    list(executor.map(lambda index: self.api.add_pet(
+                        self.pet, _headers=headers, _request_auth=auth(index)
+                    ), range(20)))
+            await asyncio.to_thread(run_calls)
+        self.assertCountEqual(
+            [request['headers']['authorization'] for request in self.requests],
+            [f'Bearer request-{index}' for index in range(20)],
+        )
+        await self.call()
+        self.assert_header('authorization', 'Bearer configured-token')
+        self.assertEqual(headers, {'content-type': 'application/json', 'authorization': 'caller'})
+        self.assertEqual(self.client.default_headers, defaults)
+
+    async def test_transport_reads_lowercase_content_type(self):
+        headers = {'content-type': 'application/x-www-form-urlencoded'}
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('content-type', 'application/x-www-form-urlencoded')
+        self.assertFalse(self.requests[-1].get('encode_multipart', True))
+        self.assertEqual(self.requests[-1]['fields'], [('field', 'value')])
+        self.assertEqual(headers, {'content-type': 'application/x-www-form-urlencoded'})
+
+    async def test_transport_reads_lowercase_content_type_and_preserves_repeated_fields(self):
+        import urllib3
+        headers = urllib3.HTTPHeaderDict({'content-type': 'application/x-www-form-urlencoded'})
+        headers.add('x-list', 'a')
+        headers.add('x-list', 'b')
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/form', headers=headers, post_params=[('field', 'value')]
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assert_header('content-type', 'application/x-www-form-urlencoded')
+        self.assertEqual(self.values(self.requests[-1]['headers'], 'x-list'), ['a', 'b'])
+        self.assertEqual(self.values(headers, 'x-list'), ['a', 'b'])
+        self.assertFalse(self.requests[-1]['encode_multipart'])
+        self.assertEqual(self.requests[-1]['fields'], [('field', 'value')])
+
+    async def test_transport_multipart_removes_content_type_without_mutating_input(self):
+        headers = {'content-type': 'multipart/form-data'}
+        result = self.client.rest_client.request(
+            'POST', 'https://example.test/upload', headers=headers,
+            post_params=[('file', ('name.txt', b'content', 'text/plain'))],
+        )
+        if inspect.isawaitable(result):
+            await result
+        self.assertNotIn('content-type', self.requests[-1]['headers'])
+        self.assertEqual(headers, {'content-type': 'multipart/form-data'})