diff --git a/sagemaker-serve/src/sagemaker/serve/model_builder.py b/sagemaker-serve/src/sagemaker/serve/model_builder.py
index 24e747f206..0f5400b13c 100644
--- a/sagemaker-serve/src/sagemaker/serve/model_builder.py
+++ b/sagemaker-serve/src/sagemaker/serve/model_builder.py
@@ -599,6 +599,10 @@ def _initialize_defaults(self) -> None:
             self.framework = None
         if not hasattr(self, "framework_version"):
             self.framework_version = None
+        if not hasattr(self, "content_types"):
+            self.content_types = None
+        if not hasattr(self, "response_types"):
+            self.response_types = None
 
     def _fetch_default_instance_type_for_custom_model(self) -> str:
         hosting_configs = self._fetch_hosting_configs_for_custom_model()
diff --git a/sagemaker-serve/tests/unit/test_model_builder_register.py b/sagemaker-serve/tests/unit/test_model_builder_register.py
new file mode 100644
index 0000000000..14195e445d
--- /dev/null
+++ b/sagemaker-serve/tests/unit/test_model_builder_register.py
@@ -0,0 +1,78 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"). You
+# may not use this file except in compliance with the License. A copy of
+# the License is located at
+#
+#     http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file is
+# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
+# ANY KIND, either express or implied. See the License for the specific
+# language governing permissions and limitations under the License.
+"""Unit tests for ModelBuilder.register()."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from sagemaker.serve.model_builder import ModelBuilder
+
+ROLE_ARN = "arn:aws:iam::123456789012:role/SageMakerRole"
+IMAGE_URI = "123456789012.dkr.ecr.us-west-2.amazonaws.com/inference:latest"
+MODEL_PACKAGE_ARN = "arn:aws:sagemaker:us-west-2:123456789012:model-package/group/1"
+
+
+@pytest.fixture
+def model_builder():
+    session = MagicMock(boto_region_name="us-west-2")
+    # register() is telemetry-wrapped and reads sagemaker_config.
+    session.sagemaker_config = {}
+    return ModelBuilder(
+        image_uri=IMAGE_URI,
+        s3_model_data_url="s3://bucket/model.tar.gz",
+        role_arn=ROLE_ARN,
+        sagemaker_session=session,
+        instance_type="ml.m5.large",
+    )
+
+
+def _register(model_builder, **kwargs):
+    """Call register() and return the arguments it passes to create the model package."""
+    with patch(
+        "sagemaker.serve.model_builder.create_model_package_from_containers",
+        return_value={"ModelPackageArn": MODEL_PACKAGE_ARN},
+    ) as create_model_package, patch.object(
+        model_builder, "_prepare_container_def", return_value={"Image": IMAGE_URI}
+    ):
+        arn = model_builder.register(model_package_group_name="group", **kwargs)
+    assert arn == MODEL_PACKAGE_ARN
+    return create_model_package.call_args.kwargs
+
+
+@pytest.mark.parametrize(
+    "types",
+    [
+        {},
+        {"content_types": ["text/csv"]},
+        {"response_types": ["application/json"]},
+    ],
+    ids=["neither", "content_types_only", "response_types_only"],
+)
+def test_register_without_content_or_response_types(model_builder, types):
+    request_args = _register(model_builder, **types)
+
+    for name in ("content_types", "response_types"):
+        if name in types:
+            assert request_args[name] == types[name]
+        else:
+            assert name not in request_args
+
+
+def test_register_reuses_content_and_response_types_from_earlier_call(model_builder):
+    _register(model_builder, content_types=["text/csv"], response_types=["application/json"])
+
+    request_args = _register(model_builder)
+
+    assert request_args["content_types"] == ["text/csv"]
+    assert request_args["response_types"] == ["application/json"]