fix: Flush pending transcriptions on turn/generation complete or interrupt for Gemini API

The Gemini API may not always send an explicit transcription finished signal. This change ensures that any buffered input or output transcription text is yielded as a finished transcription when a turn is completed, generation is complete, or the session is interrupted.

Also, refined the check for `event.partial` in runners.py to be more explicit.

Co-authored-by: Hangfei Lin <[email protected]>
PiperOrigin-RevId: 839008606

Author: hangfei

contributing/samples/live_bidi_streaming_single_agent/agent.py

@@ -65,8 +65,8 @@ async def check_prime(nums: list[int]) -> str:
 
 
 root_agent = Agent(
-    # model='gemini-live-2.5-flash-preview-native-audio-09-2025',  # vertex
-    model='gemini-2.5-flash-native-audio-preview-09-2025',  # for AI studio
+    model='gemini-live-2.5-flash-preview-native-audio-09-2025',  # vertex
+    # model='gemini-2.5-flash-native-audio-preview-09-2025',  # for AI studio
     # key
     name='roll_dice_agent',
     description=(

src/google/adk/models/gemini_llm_connection.py

@@ -21,6 +21,7 @@
 from google.genai import types
 
 from ..utils.context_utils import Aclosing
+from ..utils.variant_utils import GoogleLLMVariant
 from .base_llm_connection import BaseLlmConnection
 from .llm_response import LlmResponse
 
@@ -36,10 +37,15 @@
 class GeminiLlmConnection(BaseLlmConnection):
   """The Gemini model connection."""
 
-  def __init__(self, gemini_session: live.AsyncSession):
+  def __init__(
+      self,
+      gemini_session: live.AsyncSession,
+      api_backend: GoogleLLMVariant = GoogleLLMVariant.VERTEX_AI,
+  ):
     self._gemini_session = gemini_session
     self._input_transcription_text: str = ''
     self._output_transcription_text: str = ''
+    self._api_backend = api_backend
 
   async def send_history(self, history: list[types.Content]):
     """Sends the conversation history to the gemini model.
@@ -171,6 +177,9 @@ async def receive(self) -> AsyncGenerator[LlmResponse, None]:
               yield self.__build_full_text_response(text)
               text = ''
             yield llm_response
+          # Note: in some cases, tool_call may arrive before
+          # generation_complete, causing transcription to appear after
+          # tool_call in the session log.
           if message.server_content.input_transcription:
             if message.server_content.input_transcription.text:
               self._input_transcription_text += (
@@ -215,6 +224,32 @@ async def receive(self) -> AsyncGenerator[LlmResponse, None]:
                   partial=False,
               )
               self._output_transcription_text = ''
+          # The Gemini API might not send a transcription finished signal.
+          # Instead, we rely on generation_complete, turn_complete or
+          # interrupted signals to flush any pending transcriptions.
+          if self._api_backend == GoogleLLMVariant.GEMINI_API and (
+              message.server_content.interrupted
+              or message.server_content.turn_complete
+              or message.server_content.generation_complete
+          ):
+            if self._input_transcription_text:
+              yield LlmResponse(
+                  input_transcription=types.Transcription(
+                      text=self._input_transcription_text,
+                      finished=True,
+                  ),
+                  partial=False,
+              )
+              self._input_transcription_text = ''
+            if self._output_transcription_text:
+              yield LlmResponse(
+                  output_transcription=types.Transcription(
+                      text=self._output_transcription_text,
+                      finished=True,
+                  ),
+                  partial=False,
+              )
+              self._output_transcription_text = ''
           if message.server_content.turn_complete:
             if text:
               yield self.__build_full_text_response(text)

src/google/adk/models/google_llm.py

@@ -342,7 +342,7 @@ async def connect(self, llm_request: LlmRequest) -> BaseLlmConnection:
     async with self._live_api_client.aio.live.connect(
         model=llm_request.model, config=llm_request.live_connect_config
     ) as live_session:
-      yield GeminiLlmConnection(live_session)
+      yield GeminiLlmConnection(live_session, api_backend=self._api_backend)
 
   async def _adapt_computer_use_tool(self, llm_request: LlmRequest) -> None:
     """Adapt the google computer use predefined functions to the adk computer use toolset."""

src/google/adk/runners.py

@@ -67,6 +67,23 @@
 logger = logging.getLogger('google_adk.' + __name__)
 
 
+def _is_tool_call_or_response(event: Event) -> bool:
+  return bool(event.get_function_calls() or event.get_function_responses())
+
+
+def _is_transcription(event: Event) -> bool:
+  return (
+      event.input_transcription is not None
+      or event.output_transcription is not None
+  )
+
+
+def _has_non_empty_transcription_text(transcription) -> bool:
+  return bool(
+      transcription and transcription.text and transcription.text.strip()
+  )
+
+
 class Runner:
   """The Runner class is used to run agents.
 
@@ -626,6 +643,7 @@ async def _exec_with_plugin(
       invocation_context: The invocation context
       session: The current session
       execute_fn: A callable that returns an AsyncGenerator of Events
+      is_live_call: Whether this is a live call
 
     Yields:
       Events from the execution, including any generated by plugins
@@ -651,13 +669,74 @@ async def _exec_with_plugin(
       yield early_exit_event
     else:
       # Step 2: Otherwise continue with normal execution
+      # Note for live/bidi:
+      # the transcription may arrive later then the action(function call
+      # event and thus function response event). In this case, the order of
+      # transcription and function call event will be wrong if we just
+      # append as it arrives. To address this, we should check if there is
+      # transcription going on. If there is transcription going on, we
+      # should hold on appending the function call event until the
+      # transcription is finished. The transcription in progress can be
+      # identified by checking if the transcription event is partial. When
+      # the next transcription event is not partial, it means the previous
+      # transcription is finished. Then if there is any buffered function
+      # call event, we should append them after this finished(non-parital)
+      # transcription event.
+      buffered_events: list[Event] = []
+      is_transcribing: bool = False
+
       async with Aclosing(execute_fn(invocation_context)) as agen:
         async for event in agen:
-          if not event.partial:
-            if self._should_append_event(event, is_live_call):
+          if is_live_call:
+            if event.partial and _is_transcription(event):
+              is_transcribing = True
+            if is_transcribing and _is_tool_call_or_response(event):
+              # only buffer function call and function response event which is
+              # non-partial
+              buffered_events.append(event)
+              continue
+            # Note for live/bidi: for audio response, it's considered as
+            # non-paritla event(event.partial=None)
+            # event.partial=False and event.partial=None are considered as
+            # non-partial event; event.partial=True is considered as partial
+            # event.
+            if event.partial is not True:
+              if _is_transcription(event) and (
+                  _has_non_empty_transcription_text(event.input_transcription)
+                  or _has_non_empty_transcription_text(
+                      event.output_transcription
+                  )
+              ):
+                # transcription end signal, append buffered events
+                is_transcribing = False
+                logger.debug(
+                    'Appending transcription finished event: %s', event
+                )
+                if self._should_append_event(event, is_live_call):
+                  await self.session_service.append_event(
+                      session=session, event=event
+                  )
+
+                for buffered_event in buffered_events:
+                  logger.debug('Appending buffered event: %s', buffered_event)
+                  await self.session_service.append_event(
+                      session=session, event=buffered_event
+                  )
+                buffered_events = []
+              else:
+                # non-transcription event or empty transcription event, for
+                # example, event that stores blob reference, should be appended.
+                if self._should_append_event(event, is_live_call):
+                  logger.debug('Appending non-buffered event: %s', event)
+                  await self.session_service.append_event(
+                      session=session, event=event
+                  )
+          else:
+            if event.partial is not True:
               await self.session_service.append_event(
                   session=session, event=event
               )
+
           # Step 3: Run the on_event callbacks to optionally modify the event.
           modified_event = await plugin_manager.run_on_event_callback(
               invocation_context=invocation_context, event=event