diff --git a/google/genai/models.py b/google/genai/models.py
index 4a62a917e..dc4365854 100644
--- a/google/genai/models.py
+++ b/google/genai/models.py
@@ -37,6 +37,44 @@
 logger = logging.getLogger('google_genai.models')
 
 
+def _filter_thought_parts(
+    return_value: 'types.GenerateContentResponse',
+    parameter_model: 'types._GenerateContentParameters',
+) -> 'types.GenerateContentResponse':
+  """Filters thought parts from response when include_thoughts is False.
+
+  When the Vertex AI API returns thought parts despite include_thoughts=False
+  being set in ThinkingConfig, this function performs client-side filtering
+  to suppress them. The part.thought flag is reliably set by the API, so
+  filtering on it is safe.
+
+  Args:
+    return_value: The GenerateContentResponse to filter.
+    parameter_model: The request parameters, used to read ThinkingConfig.
+
+  Returns:
+    The response with thought parts removed if include_thoughts=False,
+    otherwise the response unchanged.
+  """
+  config = parameter_model.config
+  if config is None:
+    return return_value
+  thinking_config = getattr(config, 'thinking_config', None)
+  if thinking_config is None:
+    return return_value
+  include_thoughts = getattr(thinking_config, 'include_thoughts', None)
+  if include_thoughts is not False:
+    return return_value
+  if not return_value.candidates:
+    return return_value
+  for candidate in return_value.candidates:
+    if candidate.content and candidate.content.parts:
+      candidate.content.parts = [
+          part for part in candidate.content.parts if not part.thought
+      ]
+  return return_value
+
+
 def _PersonGeneration_to_mldev_enum_validate(enum_value: Any) -> None:
   if enum_value in set(['ALLOW_ALL']):
     raise ValueError(f'{enum_value} enum value is not supported in Gemini API.')
@@ -4725,7 +4763,7 @@ def _generate_content(
         headers=response.headers
     )
     self._api_client._verify_response(return_value)
-    return return_value
+    return _filter_thought_parts(return_value, parameter_model)
 
   def _generate_content_stream(
       self,
@@ -4826,7 +4864,7 @@ def _generate_content_stream(
           headers=response.headers
       )
       self._api_client._verify_response(return_value)
-      yield return_value
+      yield _filter_thought_parts(return_value, parameter_model)
 
   def _embed_content(
       self,
@@ -6891,7 +6929,7 @@ async def _generate_content(
         headers=response.headers
     )
     self._api_client._verify_response(return_value)
-    return return_value
+    return _filter_thought_parts(return_value, parameter_model)
 
   async def _generate_content_stream(
       self,
@@ -6995,7 +7033,7 @@ async def async_generator():  # type: ignore[no-untyped-def]
             headers=response.headers
         )
         self._api_client._verify_response(return_value)
-        yield return_value
+        yield _filter_thought_parts(return_value, parameter_model)
 
     return async_generator()  # type: ignore[no-untyped-call, no-any-return]
 
diff --git a/google/genai/tests/models/test_filter_thought_parts.py b/google/genai/tests/models/test_filter_thought_parts.py
new file mode 100644
index 000000000..7e07509d3
--- /dev/null
+++ b/google/genai/tests/models/test_filter_thought_parts.py
@@ -0,0 +1,139 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License 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 _filter_thought_parts.
+
+Verifies client-side filtering of thought parts when include_thoughts=False
+is set in ThinkingConfig. Regression test for:
+https://github.com/googleapis/python-genai/issues/2239
+"""
+
+import pytest
+
+from ... import _transformers as t
+from ... import types
+from ...models import _filter_thought_parts
+
+
+def _make_response_with_thoughts() -> types.GenerateContentResponse:
+  """Build a synthetic response resembling Vertex AI image gen with thoughts."""
+  parts = [
+      types.Part(text='thinking step 1', thought=True),
+      types.Part(text='thinking step 2', thought=True),
+      types.Part(inline_data=types.Blob(mime_type='image/png', data=b'draft'), thought=True),
+      types.Part(text='self-critique', thought=True),
+      types.Part(inline_data=types.Blob(mime_type='image/png', data=b'final')),
+  ]
+  content = types.Content(role='model', parts=parts)
+  candidate = types.Candidate(content=content)
+  return types.GenerateContentResponse(candidates=[candidate])
+
+
+def _make_parameter_model(include_thoughts) -> types._GenerateContentParameters:
+  return types._GenerateContentParameters(
+      model='gemini-3.1-flash-image-preview',
+      contents=t.t_contents('Draw a red car'),
+      config=types.GenerateContentConfig(
+          thinking_config=types.ThinkingConfig(include_thoughts=include_thoughts)
+      ),
+  )
+
+
+class TestFilterThoughtParts:
+
+  def test_include_thoughts_false_removes_thought_parts(self):
+    """When include_thoughts=False, all parts with thought=True are removed."""
+    response = _make_response_with_thoughts()
+    parameter_model = _make_parameter_model(include_thoughts=False)
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    parts = result.candidates[0].content.parts
+    assert all(not part.thought for part in parts), (
+        'Expected no thought parts but found some'
+    )
+    assert len(parts) == 1, f'Expected 1 non-thought part, got {len(parts)}'
+    assert parts[0].inline_data is not None
+    assert parts[0].inline_data.data == b'final'
+
+  def test_include_thoughts_true_preserves_all_parts(self):
+    """When include_thoughts=True, no parts are filtered."""
+    response = _make_response_with_thoughts()
+    parameter_model = _make_parameter_model(include_thoughts=True)
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    parts = result.candidates[0].content.parts
+    assert len(parts) == 5, f'Expected 5 parts, got {len(parts)}'
+
+  def test_include_thoughts_none_preserves_all_parts(self):
+    """When include_thoughts is unset, no parts are filtered."""
+    response = _make_response_with_thoughts()
+    parameter_model = _make_parameter_model(include_thoughts=None)
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    parts = result.candidates[0].content.parts
+    assert len(parts) == 5
+
+  def test_no_thinking_config_preserves_all_parts(self):
+    """When ThinkingConfig is absent entirely, no parts are filtered."""
+    response = _make_response_with_thoughts()
+    parameter_model = types._GenerateContentParameters(
+        model='gemini-3.1-flash-image-preview',
+        contents=t.t_contents('Draw a red car'),
+        config=types.GenerateContentConfig(),
+    )
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    parts = result.candidates[0].content.parts
+    assert len(parts) == 5
+
+  def test_no_config_preserves_all_parts(self):
+    """When config is None entirely, no parts are filtered."""
+    response = _make_response_with_thoughts()
+    parameter_model = types._GenerateContentParameters(
+        model='gemini-3.1-flash-image-preview',
+        contents=t.t_contents('Draw a red car'),
+    )
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    parts = result.candidates[0].content.parts
+    assert len(parts) == 5
+
+  def test_empty_candidates_is_safe(self):
+    """Response with no candidates does not raise."""
+    response = types.GenerateContentResponse(candidates=[])
+    parameter_model = _make_parameter_model(include_thoughts=False)
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    assert result.candidates == []
+
+  def test_no_thought_parts_in_response(self):
+    """If API returns no thought parts, filtering is a no-op."""
+    parts = [
+        types.Part(inline_data=types.Blob(mime_type='image/png', data=b'final')),
+    ]
+    content = types.Content(role='model', parts=parts)
+    candidate = types.Candidate(content=content)
+    response = types.GenerateContentResponse(candidates=[candidate])
+    parameter_model = _make_parameter_model(include_thoughts=False)
+
+    result = _filter_thought_parts(response, parameter_model)
+
+    assert len(result.candidates[0].content.parts) == 1