fix(sessions): drop unsupported part_metadata before Vertex appendEvent

src/google/adk/sessions/vertex_ai_session_service.py

@@ -78,6 +78,21 @@ def _set_internal_custom_metadata(
   }
 
 
+def _drop_vertex_unsupported_part_fields(content_dict: dict[str, Any]) -> None:
+  """Drops Part fields the Vertex AI Agent Engine Sessions API rejects.
+
+  ``part_metadata`` is a Gemini Developer API-only field (the model path guards
+  it in ``genai`` ``_Part_to_vertex``); the Agent Engine Sessions API does not
+  accept it and fails ``appendEvent`` with ``400 INVALID_ARGUMENT`` ("Unknown
+  name \"part_metadata\" at 'event.content.parts[0]'"). Mutates the serialized
+  content dict in place; tolerant of either field-name or alias serialization.
+  """
+  for part in content_dict.get('parts') or []:
+    if isinstance(part, dict):
+      part.pop('part_metadata', None)
+      part.pop('partMetadata', None)
+
+
 class VertexAiSessionService(BaseSessionService):
   """Connects to the Vertex AI Agent Engine Session Service using Agent Engine SDK.
 
@@ -339,9 +354,9 @@ async def append_event(self, session: Session, event: Event) -> Event:
     # Build config (Monolithic approach)
     config = {}
     if event.content:
-      config['content'] = event.content.model_dump(
-          exclude_none=True, mode='json'
-      )
+      content_dict = event.content.model_dump(exclude_none=True, mode='json')
+      _drop_vertex_unsupported_part_fields(content_dict)
+      config['content'] = content_dict
     if event.actions:
       config['actions'] = {
           'skip_summarization': event.actions.skip_summarization,
@@ -406,6 +421,8 @@ async def append_event(self, session: Session, event: Event) -> Event:
         mode='json',
         by_alias=True,
     )
+    if isinstance(config['raw_event'].get('content'), dict):
+      _drop_vertex_unsupported_part_fields(config['raw_event']['content'])
 
     # Retry without raw_event if client side validation fails for older SDK
     # versions.

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -1088,6 +1088,86 @@ async def test_append_event():
   assert retrieved_session.events[1] == event_to_append
 
 
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_append_event_strips_unsupported_part_metadata(
+    mock_api_client_instance: MockAsyncClient,
+) -> None:
+  """part_metadata must not reach the Sessions API (#6014).
+
+  ``Part.part_metadata`` is a Gemini Developer API-only field; the Vertex AI
+  Agent Engine Sessions ``appendEvent`` API rejects it with 400 INVALID_ARGUMENT
+  ("Unknown name \"part_metadata\""). It must be dropped from both the
+  ``content`` and ``raw_event`` payloads, while the part text is preserved.
+  """
+  session_service = mock_vertex_ai_session_service()
+  session = await session_service.get_session(
+      app_name='123', user_id='user', session_id='1'
+  )
+  event_to_append = Event(
+      invocation_id='inv_part_metadata',
+      author='user',
+      timestamp=1734005533.0,
+      content=genai_types.Content(
+          parts=[
+              genai_types.Part(
+                  text='hello', part_metadata={'source': 'portal'}
+              ),
+              genai_types.Part(text='world', part_metadata={'n': 1}),
+          ],
+      ),
+  )
+
+  await session_service.append_event(session, event_to_append)
+
+  appended = mock_api_client_instance.event_dict['1'][0][-1]
+  for part in appended['content']['parts']:
+    assert 'part_metadata' not in part
+    assert 'partMetadata' not in part
+  for part in appended['raw_event']['content']['parts']:
+    assert 'part_metadata' not in part
+    assert 'partMetadata' not in part
+  assert [p['text'] for p in appended['content']['parts']] == ['hello', 'world']
+
+
+@pytest.mark.asyncio
+@pytest.mark.usefixtures('mock_get_api_client')
+async def test_append_event_with_part_metadata_round_trips(
+    mock_api_client_instance: MockAsyncClient,
+) -> None:
+  """Reconstruction side of #6014: an event carrying part_metadata appends and
+  reads back without error. part_metadata is dropped (unsupported on Vertex),
+  but the session round-trips and the part text is preserved.
+  """
+  session_service = mock_vertex_ai_session_service()
+  session = await session_service.get_session(
+      app_name='123', user_id='user', session_id='1'
+  )
+  event_to_append = Event(
+      invocation_id='inv_part_metadata_rt',
+      author='user',
+      timestamp=1734005533.0,
+      content=genai_types.Content(
+          role='user',
+          parts=[
+              genai_types.Part(text='hello', part_metadata={'source': 'portal'})
+          ],
+      ),
+  )
+
+  await session_service.append_event(session, event_to_append)
+  retrieved = await session_service.get_session(
+      app_name='123', user_id='user', session_id='1'
+  )
+
+  appended = next(
+      e for e in retrieved.events if e.invocation_id == 'inv_part_metadata_rt'
+  )
+  assert appended.content is not None
+  assert appended.content.parts[0].text == 'hello'
+  assert appended.content.parts[0].part_metadata is None
+
+
 @pytest.mark.asyncio
 @pytest.mark.usefixtures('mock_get_api_client')
 async def test_append_event_with_compaction():