fix: Event.message honors subclass field

_accept_convenience_kwargs already routes construction kwargs to a
subclass-declared `message` field, but the property/setter always used
`content`, so reads of that field returned the content alias instead of
the stored value. Defer to the field when present; base Event behavior
(message aliases content) is unchanged.

Change-Id: I8dd492407b8117ad9c6a90fb24269a03dac49ac0

Author: GWeale

src/google/adk/events/event.py

@@ -230,13 +230,30 @@ def _accept_convenience_kwargs(cls, data: Any) -> Any:
     return data
 
   @property
-  def message(self) -> Optional[types.Content]:
-    """Alias for content. Returns the user-facing message of the event."""
+  def message(self) -> Any:
+    """Alias for content. Returns the user-facing message of the event.
+
+    Subclasses may declare ``message`` as a real field (see
+    ``_accept_convenience_kwargs``, which already routes construction kwargs to
+    such fields). When they do, return that field's value so reads stay
+    consistent with construction and serialization instead of returning the
+    ``content`` alias. The return type is ``Any`` because such a subclass field
+    may be typed differently (e.g. ``str``); for a plain ``Event`` this returns
+    ``Optional[types.Content]``.
+    """
+    if 'message' in type(self).model_fields:
+      return self.__dict__.get('message')
     return self.content
 
   @message.setter
-  def message(self, value: Optional[types.ContentUnion]) -> None:
-    """Sets the content of the event."""
+  def message(self, value: Any) -> None:
+    """Sets the content of the event (or a subclass ``message`` field)."""
+    if 'message' in type(self).model_fields:
+      # Route through Pydantic so a subclass field's validators/type are
+      # enforced. A direct __dict__ write would skip validation, and
+      # object.__setattr__/self.message would recurse through this property.
+      self.__pydantic_validator__.validate_assignment(self, 'message', value)
+      return
     if value is not None:
       from google.genai import _transformers
 

tests/unittests/events/test_event_message.py

@@ -137,3 +137,42 @@ def test_message_with_route(self):
     event = Event(message='hello', route='next')
     assert event.content is not None
     assert event.actions.route == 'next'
+
+
+class TestMessageSubclassField:
+  """Tests that a subclass declaring `message` as a real field is honored.
+
+  `_accept_convenience_kwargs` already routes construction kwargs to such a
+  field; the `message` property/setter must defer to it too instead of
+  aliasing `content`.
+  """
+
+  def test_subclass_field_readable_via_property(self):
+    class _Sub(Event):
+      message: str = ''
+
+    event = _Sub(message='hello', author='a')
+    assert event.message == 'hello'
+
+  def test_subclass_field_serializes_and_round_trips(self):
+    class _Sub(Event):
+      message: str = ''
+
+    event = _Sub(message='hello', author='a')
+    data = event.model_dump()
+    assert data['message'] == 'hello'
+    assert _Sub.model_validate(data).message == 'hello'
+
+  def test_subclass_field_setter_updates_field_not_content(self):
+    class _Sub(Event):
+      message: str = ''
+
+    event = _Sub(message='hello', author='a')
+    event.message = 'updated'
+    assert event.message == 'updated'
+    assert event.content is None
+
+  def test_base_event_message_still_aliases_content(self):
+    content = types.Content(parts=[types.Part(text='hi')], role='model')
+    event = Event(content=content)
+    assert event.message is event.content