diff --git a/python/packages/core/agent_framework/_types.py b/python/packages/core/agent_framework/_types.py
index 4b8572e884a..de30bf6d67d 100644
--- a/python/packages/core/agent_framework/_types.py
+++ b/python/packages/core/agent_framework/_types.py
@@ -4215,11 +4215,19 @@ def merge_chat_options(
             # Merge tools lists
             base_tools = result.get("tools")
             if base_tools and value:
-                # Add tools that aren't already present
-                merged_tools = list(base_tools)
-                for tool in value if isinstance(value, Iterable) else [value]:  # type: ignore[reportUnknownVariableType]
-                    if tool not in merged_tools:
-                        merged_tools.append(tool)
+                # Add tools that aren't already present, matching by name so the same
+                # plain callable wrapped on both sides is not added twice.
+                from ._tools import _get_tool_name  # pyright: ignore[reportPrivateUsage]
+
+                merged_tools = normalize_tools(base_tools)
+                names = {name for t in merged_tools if (name := _get_tool_name(t))}
+                for tool in normalize_tools(value):
+                    name = _get_tool_name(tool)
+                    if tool in merged_tools or (name is not None and name in names):
+                        continue
+                    merged_tools.append(tool)
+                    if name is not None:
+                        names.add(name)
                 result["tools"] = merged_tools
             elif value:
                 result["tools"] = value if isinstance(value, list) else [value]
diff --git a/python/packages/core/tests/core/test_types.py b/python/packages/core/tests/core/test_types.py
index 585a654a3df..495069db620 100644
--- a/python/packages/core/tests/core/test_types.py
+++ b/python/packages/core/tests/core/test_types.py
@@ -5327,3 +5327,56 @@ def test_agent_response_update_serialization_includes_finish_reason() -> None:
 
 
 # endregion
+
+
+def test_merge_chat_options_single_mapping_tool_is_not_spread_into_keys():
+    """A single tool given as a mapping is one tool, on either side of the merge."""
+
+    @tool
+    def my_tool() -> None:
+        pass
+
+    hosted = {"type": "web_search", "name": "ws"}
+
+    merged = merge_chat_options({"tools": [my_tool]}, {"tools": hosted})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [my_tool, hosted]
+
+    merged = merge_chat_options({"tools": hosted}, {"tools": [my_tool]})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [hosted, my_tool]
+
+
+def test_merge_chat_options_same_plain_callable_on_both_sides():
+    """The same undecorated function on both sides is merged into one tool."""
+
+    def my_func() -> None:
+        pass
+
+    merged = merge_chat_options({"tools": [my_func]}, {"tools": [my_func]})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert [t.name for t in merged["tools"]] == ["my_func"]
+
+
+def test_merge_chat_options_keeps_pydantic_provider_tool_whole():
+    """A Pydantic provider-native tool spec is one tool, not spread into its fields."""
+
+    class ProviderTool(BaseModel):
+        type: str = "code_interpreter"
+        container: str = "auto"
+
+    @tool
+    def my_tool() -> None:
+        pass
+
+    provider_tool = ProviderTool()
+
+    merged = merge_chat_options({"tools": [my_tool]}, {"tools": [provider_tool]})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [my_tool, provider_tool]
+
+    merged = merge_chat_options({"tools": [provider_tool]}, {"tools": [my_tool]})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [provider_tool, my_tool]
+
+    # A single, unwrapped provider tool on either side.
+    merged = merge_chat_options({"tools": [my_tool]}, {"tools": provider_tool})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [my_tool, provider_tool]
+
+    merged = merge_chat_options({"tools": provider_tool}, {"tools": [my_tool]})  # type: ignore[arg-type]  # pyrefly: ignore[bad-argument-type]  # ty: ignore[invalid-argument-type]
+    assert merged["tools"] == [provider_tool, my_tool]