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

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Bug Fixes
+
+* fix(agents): tolerate empty A2A message parts in RemoteA2aAgent; add unit test ([fix/a2a-handle-empty-parts])
+
 ## [1.20.0](https://github.com/google/adk-python/compare/v1.19.0...v1.20.0) (2025-12-01)
 
 
@@ -40,7 +46,6 @@
 
 * Add Code Wiki badge to README ([caf23ac](https://github.com/google/adk-python/commit/caf23ac49fe08bc7f625c61eed4635c26852c3ba))
 
-
 ## [1.19.0](https://github.com/google/adk-python/compare/v1.18.0...v1.19.0) (2025-11-19)
 
 ### Features

src/google/adk/agents/remote_a2a_agent.py

@@ -412,8 +412,14 @@ 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

@@ -1010,6 +1010,45 @@ 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."""