[bugfix] Fix bug with exploding agent runs due to broken conversion of content Parts to LiteLLM messages format

src/google/adk/models/lite_llm.py

@@ -470,6 +470,8 @@ def _part_has_payload(part: types.Part) -> bool:
     return True
   if part.file_data and (part.file_data.file_uri or part.file_data.data):
     return True
+  if part.function_response:
+    return True
   return False
 
 

tests/unittests/models/test_litellm.py

@@ -990,6 +990,47 @@ async def test_generate_content_async_adds_fallback_user_message(
   )
 
 
+@pytest.mark.asyncio
+async def test_generate_content_async_no_fallback_for_function_response(
+    mock_acompletion, lite_llm_instance
+):
+  """Tests that no fallback message is added for a user message with a function response."""
+  llm_request = LlmRequest(
+      contents=[
+          types.Content(
+              role="user",
+              parts=[
+                  types.Part.from_function_response(
+                      name="test_function",
+                      response={"result": "test_result"},
+                  )
+              ],
+          )
+      ]
+  )
+
+  # Run generate_content_async which calls _append_fallback_user_content_if_missing
+  async for _ in lite_llm_instance.generate_content_async(llm_request):
+    pass
+
+  # Verify that the fallback message was NOT added to the llm_request
+  assert len(llm_request.contents) == 1
+  assert len(llm_request.contents[0].parts) == 1
+  assert llm_request.contents[0].parts[0].function_response is not None
+
+  # Verify that the message sent to litellm does not contain the fallback text
+  mock_acompletion.assert_called_once()
+  _, kwargs = mock_acompletion.call_args
+  user_messages = [
+      message for message in kwargs["messages"] if message["role"] == "user"
+  ]
+  assert not any(
+      message.get("content")
+      == "Handle the requests as specified in the System Instruction."
+      for message in user_messages
+  )
+
+
 litellm_append_user_content_test_cases = [
     pytest.param(
         LlmRequest(