fix: Preserve event details when output is delegated

When a node delegates its output, the event is now enqueued with the `output` field set to None, while other event details such as actions and content are preserved. Previously, the entire event was skipped, leading to loss of information.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 930758396

Author: DeanChensj

src/google/adk/workflow/_node_runner.py

@@ -40,6 +40,13 @@
 logger = logging.getLogger("google_adk." + __name__)
 
 
+def _has_non_output_content(event: Event) -> bool:
+  if event.actions:
+    if event.actions.state_delta or event.actions.artifact_delta:
+      return True
+  return False
+
+
 class NodeRunner:
   """Per-node executor. Drives BaseNode.run(), enriches events.
 
@@ -314,12 +321,14 @@ def _track_event_in_context(self, event: Event, ctx: Context) -> None:
   async def _enqueue_event(self, event: Event, ctx: Context) -> None:
     """Enrich and enqueue event to the session.
 
-    Skips enqueueing if output is delegated via use_as_output —
-    the child already emitted it. Pending deltas stay in ctx for
-    _flush_output_and_deltas.
+    Suppresses output if output is delegated via use_as_output (since the child
+    already emitted it), but preserves other event details. Pending deltas stay
+    in ctx for _flush_output_and_deltas.
     """
     if event.output is not None and ctx._output_delegated:
-      return
+      if not _has_non_output_content(event):
+        return
+      event = event.model_copy(update={"output": None})
 
     self._enrich_event(event, ctx)
     if not event.partial:

tests/unittests/workflow/test_node_runner_ctx.py

@@ -396,6 +396,34 @@ async def _run_impl(self, *, ctx, node_input):
   assert len(output_events) == 0
 
 
+@pytest.mark.asyncio
+async def test_delegated_output_preserves_event_details():
+  """When output is delegated, the event is enqueued but output is suppressed."""
+  from google.adk.events.event_actions import EventActions
+
+  class _Node(BaseNode):
+
+    async def _run_impl(self, *, ctx, node_input):
+      ctx._output_delegated = True
+      yield Event(
+          output='delegated_value',
+          actions=EventActions(state_delta={'foo': 'bar'}),
+          content=types.Content(role='model', parts=[types.Part(text='hello')]),
+      )
+
+  parent_ctx, events = _make_ctx()
+  child_ctx = await NodeRunner(
+      node=_Node(name='n'), parent_ctx=parent_ctx
+  ).run()
+
+  assert child_ctx.output == 'delegated_value'
+  assert len(events) == 1
+  event = events[0]
+  assert event.output is None
+  assert event.actions.state_delta == {'foo': 'bar'}
+  assert event.content.parts[0].text == 'hello'
+
+
 # =========================================================================
 # Context as INPUT — resume state provided to NodeRunner at construction
 # =========================================================================