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

Guard against empty event.content.parts when handling initial TASK_REQUIRED responses. Added unit test covering the empty-parts scenario.

Author: 8packcoder

src/google/adk/agents/remote_a2a_agent.py

@@ -411,8 +411,14 @@ async def _handle_a2a_response(
           )
           # for streaming task, we update the event with the task status.
           # We update the event as Thought updates.
-          if task and task.status and task.status.state == TaskState.submitted:
-            event.content.parts[0].thought = True
+          if (
+              task
+              and task.status
+              and task.status.state == TaskState.submitted
+              and event.content.parts
+          ):
+            for part in event.content.parts:
+              part.thought = True
         elif (
             isinstance(update, A2ATaskStatusUpdateEvent)
             and update.status

tests/unittests/agents/test_remote_a2a_agent.py

@@ -1009,6 +1009,45 @@ async def test_handle_a2a_response_with_partial_artifact_update(self):
 
     assert result is None
 
+  @pytest.mark.asyncio
+  async def test_handle_a2a_response_with_task_submitted_and_empty_parts(self):
+    """Test handling of a task submitted response with empty parts."""
+    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 a proper Event mock that can handle custom_metadata
+    # Content with empty parts
+    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
+
+      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,
+      )
+      # 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
+
 
 class TestRemoteA2aAgentMessageHandlingFromFactory:
   """Test message handling functionality."""