Fix A2A inbound converters to restore all event metadata fields

The inbound converters in to_adk_event.py only restored `actions` from
A2A metadata, silently dropping `custom_metadata`, `usage_metadata`,
`error_code`, and `grounding_metadata`. Additionally, `_create_event()`
returned None for metadata-only messages with no parts or actions.

Add `_extract_event_metadata()` to restore these fields from A2A metadata
and update `_create_event()` to accept and apply them. Metadata-only
messages now correctly produce a valid Event.

Author: enjoykumawat

src/google/adk/a2a/converters/to_adk_event.py

@@ -177,15 +177,17 @@ def _create_event(
     actions: Optional[EventActions] = None,
     long_running_function_ids: Optional[set[str]] = None,
     partial: bool = False,
+    event_metadata: Optional[dict[str, Any]] = None,
 ) -> Optional[Event]:
   """Creates an ADK event from parts and metadata."""
   event_actions = actions or EventActions()
-  if not output_parts and not event_actions.model_dump(
-      exclude_none=True, exclude_defaults=True
-  ):
+  has_actions = bool(
+      event_actions.model_dump(exclude_none=True, exclude_defaults=True)
+  )
+  if not output_parts and not has_actions and not event_metadata:
     return None
 
-  event = Event(
+  event_kwargs: dict[str, Any] = dict(
       invocation_id=(
           invocation_context.invocation_id
           if invocation_context
@@ -208,7 +210,10 @@ def _create_event(
       partial=partial,
   )
 
-  return event
+  if event_metadata:
+    event_kwargs.update(event_metadata)
+
+  return Event(**event_kwargs)
 
 
 def _parse_adk_metadata_value(value: Any) -> Any:
@@ -248,6 +253,74 @@ def _extract_event_actions(
     return EventActions()
 
 
+def _extract_event_metadata(
+    metadata: Optional[dict[str, Any]],
+) -> dict[str, Any]:
+  """Extracts ADK event metadata fields from A2A metadata.
+
+  Restores fields like custom_metadata, usage_metadata, error_code, and
+  grounding_metadata that were serialized into A2A metadata by the outbound
+  converter.
+
+  Args:
+    metadata: The A2A metadata dictionary.
+
+  Returns:
+    A dictionary of event keyword arguments to apply to the Event.
+  """
+  if not metadata:
+    return {}
+
+  event_kwargs: dict[str, Any] = {}
+
+  # Simple fields that can be restored directly
+  simple_fields = ["error_code"]
+  for field_name in simple_fields:
+    raw_value = metadata.get(_get_adk_metadata_key(field_name))
+    if raw_value is not None:
+      event_kwargs[field_name] = _parse_adk_metadata_value(raw_value)
+
+  # Dict fields that need parsing
+  dict_fields = ["custom_metadata"]
+  for field_name in dict_fields:
+    raw_value = metadata.get(_get_adk_metadata_key(field_name))
+    if raw_value is not None:
+      parsed = _parse_adk_metadata_value(raw_value)
+      if isinstance(parsed, dict):
+        event_kwargs[field_name] = parsed
+      else:
+        logger.warning(
+            "Ignoring invalid ADK %s metadata of type %s",
+            field_name,
+            type(parsed).__name__,
+        )
+
+  # Pydantic model fields that need validation
+  model_fields = {
+      "usage_metadata": genai_types.GenerateContentResponseUsageMetadata,
+      "grounding_metadata": genai_types.GroundingMetadata,
+  }
+  for field_name, model_class in model_fields.items():
+    raw_value = metadata.get(_get_adk_metadata_key(field_name))
+    if raw_value is not None:
+      parsed = _parse_adk_metadata_value(raw_value)
+      if isinstance(parsed, dict):
+        try:
+          event_kwargs[field_name] = model_class(**parsed)
+        except (ValidationError, TypeError) as error:
+          logger.warning(
+              "Ignoring invalid ADK %s metadata: %s", field_name, error
+          )
+      else:
+        logger.warning(
+            "Ignoring invalid ADK %s metadata of type %s",
+            field_name,
+            type(parsed).__name__,
+        )
+
+  return event_kwargs
+
+
 def _merge_top_level_dicts(
     base: dict[str, Any], new_values: dict[str, Any]
 ) -> dict[str, Any]:
@@ -304,6 +377,7 @@ def convert_a2a_task_to_event(
 
   try:
     event_actions = EventActions()
+    event_metadata: dict[str, Any] = {}
     output_parts = []
     long_running_function_ids = set()
     if a2a_task.artifacts:
@@ -314,6 +388,7 @@ def convert_a2a_task_to_event(
         event_actions = _merge_event_actions(
             event_actions, _extract_event_actions(artifact.metadata)
         )
+        event_metadata.update(_extract_event_metadata(artifact.metadata))
       output_parts, _ = _convert_a2a_parts_to_adk_parts(
           artifact_parts, part_converter
       )
@@ -325,6 +400,9 @@ def convert_a2a_task_to_event(
           event_actions,
           _extract_event_actions(a2a_task.status.message.metadata),
       )
+      event_metadata.update(
+          _extract_event_metadata(a2a_task.status.message.metadata)
+      )
       parts, ids = _convert_a2a_parts_to_adk_parts(
           a2a_task.status.message.parts, part_converter
       )
@@ -337,6 +415,7 @@ def convert_a2a_task_to_event(
         author,
         event_actions,
         long_running_function_ids,
+        event_metadata=event_metadata or None,
     )
 
   except Exception as e:
@@ -380,6 +459,7 @@ def convert_a2a_message_to_event(
         invocation_context,
         author,
         _extract_event_actions(a2a_message.metadata),
+        event_metadata=_extract_event_metadata(a2a_message.metadata),
     )
 
   except Exception as e:
@@ -412,10 +492,15 @@ def convert_a2a_status_update_to_event(
     output_parts = []
     long_running_function_ids = set()
     event_actions = EventActions()
+    event_metadata: Optional[dict[str, Any]] = None
     if a2a_status_update.status.message:
       event_actions = _extract_event_actions(
           a2a_status_update.status.message.metadata
       )
+      event_metadata = (
+          _extract_event_metadata(a2a_status_update.status.message.metadata)
+          or None
+      )
       parts, ids = _convert_a2a_parts_to_adk_parts(
           a2a_status_update.status.message.parts, part_converter
       )
@@ -428,6 +513,7 @@ def convert_a2a_status_update_to_event(
         author,
         event_actions,
         long_running_function_ids,
+        event_metadata=event_metadata,
     )
   except Exception as e:
     logger.error("Failed to convert A2A status update to event: %s", e)
@@ -466,6 +552,10 @@ def convert_a2a_artifact_update_to_event(
         author,
         _extract_event_actions(a2a_artifact_update.artifact.metadata),
         partial=not a2a_artifact_update.last_chunk,
+        event_metadata=_extract_event_metadata(
+            a2a_artifact_update.artifact.metadata
+        )
+        or None,
     )
   except Exception as e:
     logger.error("Failed to convert A2A artifact update to event: %s", e)

tests/unittests/a2a/converters/test_to_adk.py

@@ -418,3 +418,157 @@ def test_convert_a2a_artifact_update_to_event_none(self):
     """Test convert_a2a_artifact_update_to_event with None."""
     with pytest.raises(ValueError, match="A2A artifact update cannot be None"):
       convert_a2a_artifact_update_to_event(None)
+
+  def test_convert_a2a_message_to_event_restores_custom_metadata(self):
+    """Test A2A message conversion restores custom_metadata."""
+    a2a_part = Mock(spec=A2APart)
+    a2a_part.root = Mock(spec=TextPart)
+    a2a_part.root.metadata = {}
+    message = Message(
+        message_id="msg-1",
+        role="user",
+        parts=[a2a_part],
+        metadata={
+            _get_adk_metadata_key("custom_metadata"): {
+                "my_key": "my_value",
+            }
+        },
+    )
+
+    mock_genai_part = genai_types.Part.from_text(text="hello")
+    mock_part_converter = Mock(return_value=[mock_genai_part])
+
+    event = convert_a2a_message_to_event(
+        message,
+        author="test-author",
+        invocation_context=self.mock_context,
+        part_converter=mock_part_converter,
+    )
+
+    assert event is not None
+    assert event.custom_metadata == {"my_key": "my_value"}
+
+  def test_convert_a2a_message_to_event_restores_error_code(self):
+    """Test A2A message conversion restores error_code."""
+    a2a_part = Mock(spec=A2APart)
+    a2a_part.root = Mock(spec=TextPart)
+    a2a_part.root.metadata = {}
+    message = Message(
+        message_id="msg-1",
+        role="user",
+        parts=[a2a_part],
+        metadata={
+            _get_adk_metadata_key("error_code"): "RESOURCE_EXHAUSTED",
+        },
+    )
+
+    mock_genai_part = genai_types.Part.from_text(text="error")
+    mock_part_converter = Mock(return_value=[mock_genai_part])
+
+    event = convert_a2a_message_to_event(
+        message,
+        author="test-author",
+        invocation_context=self.mock_context,
+        part_converter=mock_part_converter,
+    )
+
+    assert event is not None
+    assert event.error_code == "RESOURCE_EXHAUSTED"
+
+  def test_convert_a2a_message_to_event_metadata_only_returns_event(self):
+    """Test A2A message with only metadata (no parts/actions) returns event."""
+    message = Message(
+        message_id="msg-1",
+        role="user",
+        parts=[],
+        metadata={
+            _get_adk_metadata_key("custom_metadata"): {
+                "reason": "metadata-only",
+            }
+        },
+    )
+
+    event = convert_a2a_message_to_event(
+        message,
+        author="test-author",
+        invocation_context=self.mock_context,
+        part_converter=Mock(),
+    )
+
+    assert event is not None
+    assert event.custom_metadata == {"reason": "metadata-only"}
+    assert event.content is None
+
+  def test_convert_a2a_status_update_restores_custom_metadata(self):
+    """Test A2A status update conversion restores custom_metadata."""
+    a2a_part = Mock(spec=A2APart)
+    a2a_part.root = Mock(spec=TextPart)
+    a2a_part.root.metadata = {}
+    update = TaskStatusUpdateEvent(
+        task_id="task-1",
+        status=TaskStatus(
+            state=TaskState.input_required,
+            timestamp="now",
+            message=Message(
+                message_id="m1",
+                role="agent",
+                parts=[a2a_part],
+                metadata={
+                    _get_adk_metadata_key("custom_metadata"): {
+                        "trace_id": "abc-123",
+                    }
+                },
+            ),
+        ),
+        context_id="context-1",
+        final=False,
+    )
+
+    mock_genai_part = genai_types.Part.from_text(text="status text")
+    mock_part_converter = Mock(return_value=[mock_genai_part])
+
+    event = convert_a2a_status_update_to_event(
+        update,
+        author="test-author",
+        invocation_context=self.mock_context,
+        part_converter=mock_part_converter,
+    )
+
+    assert event is not None
+    assert event.custom_metadata == {"trace_id": "abc-123"}
+
+  def test_convert_a2a_task_restores_custom_metadata_from_artifact(self):
+    """Test A2A task conversion restores custom_metadata from artifact."""
+    task = Task(
+        id="task-1",
+        status=TaskStatus(
+            state=TaskState.submitted, timestamp="2024-01-01T00:00:00Z"
+        ),
+        context_id="context-1",
+        artifacts=[
+            Artifact(
+                artifact_id="art-1",
+                artifact_type="message",
+                parts=[],
+                metadata={
+                    _get_adk_metadata_key("custom_metadata"): {
+                        "task_key": "task_value",
+                    },
+                    _get_adk_metadata_key("actions"): {
+                        "stateDelta": {"saved_key": "saved-value"}
+                    },
+                },
+            )
+        ],
+    )
+
+    event = convert_a2a_task_to_event(
+        task,
+        author="test-author",
+        invocation_context=self.mock_context,
+        part_converter=Mock(),
+    )
+
+    assert event is not None
+    assert event.custom_metadata == {"task_key": "task_value"}
+    assert event.actions.state_delta == {"saved_key": "saved-value"}