fix: avoid yielding a None function-response event in live mode

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 941267314

Author: GWeale

src/google/adk/flows/llm_flows/base_llm_flow.py

@@ -1221,23 +1221,28 @@ async def _postprocess_live(
 
     # Handles function calls.
     if model_response_event.get_function_calls():
-      function_response_event = await functions.handle_function_calls_live(
+      # handle_function_calls_live returns None when every call is deferred
+      # (e.g. all long-running), so guard before yielding to avoid emitting a
+      # None event into the live stream.
+      if function_response_event := await functions.handle_function_calls_live(
           invocation_context, model_response_event, llm_request.tools_dict
-      )
-      # Always yield the function response event first
-      yield function_response_event
-
-      # Check if this is a set_model_response function response
-      if json_response := _output_schema_processor.get_structured_model_response(
-          function_response_event
       ):
-        # Create and yield a final model response event
-        final_event = (
-            _output_schema_processor.create_final_model_response_event(
-                invocation_context, json_response
+        # Always yield the function response event first
+        yield function_response_event
+
+        # Check if this is a set_model_response function response
+        if json_response := (
+            _output_schema_processor.get_structured_model_response(
+                function_response_event
             )
-        )
-        yield final_event
+        ):
+          # Create and yield a final model response event
+          final_event = (
+              _output_schema_processor.create_final_model_response_event(
+                  invocation_context, json_response
+              )
+          )
+          yield final_event
 
   async def _postprocess_run_processors_async(
       self, invocation_context: InvocationContext, llm_response: LlmResponse

tests/unittests/flows/llm_flows/test_base_llm_flow.py

@@ -1784,3 +1784,45 @@ async def test_transfer_to_sibling_from_non_llm_agent_allowed():
   # Assert
   assert agent is not None
   assert agent.name == 'child2'
+
+
+@pytest.mark.asyncio
+async def test_postprocess_live_skips_none_function_response_event():
+  """When every live function call defers, no None event must be yielded.
+
+  handle_function_calls_live returns None if all calls are long-running, and
+  yielding that None downstream crashes the live receive loop.
+  """
+  from google.adk.flows.llm_flows import base_llm_flow as blf
+
+  agent = Agent(name='test_agent', model='gemini-2.0-flash')
+  invocation_context = await testing_utils.create_invocation_context(
+      agent=agent
+  )
+  flow = BaseLlmFlowForTesting()
+
+  fc_part = types.Part(
+      function_call=types.FunctionCall(name='lro', id='1', args={})
+  )
+  content = types.Content(role='model', parts=[fc_part])
+  model_response_event = Event(
+      invocation_id=invocation_context.invocation_id,
+      author=agent.name,
+      content=content,
+  )
+  llm_request = LlmRequest(model='gemini-2.0-flash')
+  llm_response = LlmResponse(content=content)
+
+  with mock.patch.object(
+      blf.functions,
+      'handle_function_calls_live',
+      new=AsyncMock(return_value=None),
+  ):
+    events = [
+        event
+        async for event in flow._postprocess_live(
+            invocation_context, llm_request, llm_response, model_response_event
+        )
+    ]
+
+  assert all(event is not None for event in events)