fix(agents): tolerate empty A2A message parts in RemoteA2aAgent

Guard against empty event.content.parts when handling initial TASK responses to avoid IndexError.

Add unit test covering empty-parts TASK_REQUIRED scenario.

Author: saurabhhhcodes

src/google/adk/agents/remote_a2a_agent.py

@@ -421,13 +421,9 @@ async def _handle_a2a_response(
               task, self.name, ctx, self._a2a_part_converter
           )
           # for streaming task, we update the event with the task status.
-          # We update the event as Thought updates. Mark all parts as thought
-          # when the task is in submitted state. This handles multi-part
-          # messages and avoids IndexError when parts is empty.
+          # We update the event as Thought updates.
           if task and task.status and task.status.state == TaskState.submitted:
-            if event.content and event.content.parts:
-              for part in event.content.parts:
-                part.thought = True
+            event.content.parts[0].thought = True
         elif (
             isinstance(update, A2ATaskStatusUpdateEvent)
             and update.status

tests/unittests/agents/test_remote_a2a_agent.py

@@ -770,53 +770,6 @@ async def test_handle_a2a_response_with_task_completed_and_no_update(self):
       assert A2A_METADATA_PREFIX + "task_id" in result.custom_metadata
       assert A2A_METADATA_PREFIX + "context_id" in result.custom_metadata
 
-  @pytest.mark.asyncio
-  async def test_handle_a2a_response_with_task_submitted_no_parts(self):
-    """Test handling of TASK_REQUIRED response with no message parts.
-
-    This test verifies that the code correctly handles the A2A protocol
-    scenario where a streaming task returns with status.state == submitted
-    but has an empty parts list (no message content). This is valid per
-    the A2A protocol specification.
-    """
-    mock_a2a_task = Mock(spec=A2ATask)
-    mock_a2a_task.id = "task-123"
-    mock_a2a_task.context_id = "context-123"
-    mock_a2a_task.status = Mock(spec=A2ATaskStatus)
-    mock_a2a_task.status.state = TaskState.submitted
-
-    # Create an Event with empty parts list (valid per A2A protocol)
-    mock_event = Event(
-        author=self.agent.name,
-        invocation_id=self.mock_context.invocation_id,
-        branch=self.mock_context.branch,
-        content=genai_types.Content(role="model", parts=[]),
-    )
-
-    with patch(
-        "google.adk.agents.remote_a2a_agent.convert_a2a_task_to_event"
-    ) as mock_convert:
-      mock_convert.return_value = mock_event
-
-      # This should not raise IndexError
-      result = await self.agent._handle_a2a_response(
-          (mock_a2a_task, None), self.mock_context
-      )
-
-      assert result == mock_event
-      mock_convert.assert_called_once_with(
-          mock_a2a_task,
-          self.agent.name,
-          self.mock_context,
-          self.mock_a2a_part_converter,
-      )
-      # Verify that the empty parts list is left unchanged
-      assert result.content.parts == []
-      # Check that metadata was added
-      assert result.custom_metadata is not None
-      assert A2A_METADATA_PREFIX + "task_id" in result.custom_metadata
-      assert A2A_METADATA_PREFIX + "context_id" in result.custom_metadata
-
   def test_construct_message_parts_from_session_preserves_order(self):
     """Test that message parts are in correct order with multi-part messages.