fix(a2a): keep metadata-only messages in convert_a2a_message_to_event

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

@@ -661,6 +661,7 @@ def convert_a2a_message_to_event(
         _extract_event_actions(a2a_message.metadata),
         long_running_function_ids,
         content_role=content_role,
+        allow_empty_event=any(metadata_fields.values()),
         **metadata_fields,
     )
 

tests/unittests/a2a/converters/test_to_adk.py

@@ -191,6 +191,26 @@ def test_convert_a2a_message_to_event_returns_action_only_event(self):
     assert event.actions.escalate is True
     assert event.content is None
 
+  def test_convert_a2a_message_to_event_returns_metadata_only_event(self):
+    """Test A2A message conversion keeps events with only custom metadata."""
+    message = Message(
+        message_id="msg-1",
+        role=_compat.ROLE_AGENT,
+        parts=[],
+        metadata={_get_adk_metadata_key("custom_metadata"): {"flag": True}},
+    )
+
+    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 == {"flag": True}
+    assert event.content is None
+
   def test_convert_a2a_task_to_event_success(self):
     """Test successful conversion of A2A task to Event."""
     a2a_part = _make_a2a_part_for_test({})