Adding changelog description to the fix.

changelog/4178.fixed.md

@@ -0,0 +1 @@
+- Fixed timed frames (e.g. word-boundary events) arriving out of order or too late relative to TTS audio playback. When a `TTSStoppedFrame` carries a presentation timestamp, the clock queue now flushes all pending timed frames immediately once the audio task finishes sending the preceding audio, ensuring timed events always reach downstream processors before the stop signal.

src/pipecat/transports/base_output.py

@@ -359,6 +359,16 @@ class BaseOutputTransport(FrameProcessor):
             await sender.handle_mixer_control_frame(frame)
         elif isinstance(frame, TTSStoppedFrame):
             await sender.handle_sync_frame(frame)
+            if frame.pts:
+                # The frame goes into both queues. Create fresh events to keep
+                # the queues in sync: the audio task signals _clock_flush_event
+                # to trigger an immediate drain; the clock task signals
+                # _clock_drained_event once the drain is complete so the audio
+                # task knows it can safely push TTSStoppedFrame downstream.
+                # This way we can keep audio and words in sync.
+                sender._clock_flush_event = asyncio.Event()
+                sender._clock_drained_event = asyncio.Event()
+                await sender.handle_timed_frame(frame)
         elif frame.pts:
             await sender.handle_timed_frame(frame)
         else:
@@ -431,6 +441,22 @@ class BaseOutputTransport(FrameProcessor):
             self._video_task: Optional[asyncio.Task] = None
             self._clock_task: Optional[asyncio.Task] = None
 
+            # When a TTSStoppedFrame has a pts it is enqueued in both the
+            # audio_queue and the clock_queue. These two events keep the queues
+            # in sync around that frame:
+            #
+            # _clock_flush_event  – set by the audio task once all preceding
+            #   audio has been sent, telling the clock task to stop waiting on
+            #   timestamps and drain every pending frame immediately.
+            #
+            # _clock_drained_event – set by the clock task once it has drained
+            #   all frames up to and including the TTSStoppedFrame, telling the
+            #   audio task it is safe to push the TTSStoppedFrame downstream.
+            #   This guarantees that all timed frames arrive before the stop
+            #   signal regardless of which queue wins the race.
+            self._clock_flush_event: Optional[asyncio.Event] = None
+            self._clock_drained_event: Optional[asyncio.Event] = None
+
         @property
         def sample_rate(self) -> int:
             """Get the audio sample rate.
@@ -800,6 +826,26 @@ class BaseOutputTransport(FrameProcessor):
                     await self._send_silence(self._params.audio_out_end_silence_secs)
                     break
 
+                # If this TTSStoppedFrame is also in the clock queue, signal
+                # the clock task to drain immediately and then wait for it to
+                # confirm all timed frames have been pushed downstream before
+                # pushing TTSStoppedFrame here. This keeps the audio queue as
+                # the single owner of the downstream push for this frame.
+                if isinstance(frame, TTSStoppedFrame) and self._clock_flush_event is not None:
+                    logger.debug(f"{self._transport} audio queue signalling clock queue flush")
+                    self._clock_flush_event.set()
+                    try:
+                        await asyncio.wait_for(
+                            self._clock_drained_event.wait(), timeout=BOT_VAD_STOP_FALLBACK_SECS
+                        )
+                    except asyncio.TimeoutError:
+                        logger.warning(
+                            f"{self._transport} timed out waiting for clock queue to drain, "
+                            "pushing TTSStoppedFrame downstream anyway"
+                        )
+                    self._clock_flush_event = None
+                    self._clock_drained_event = None
+
                 # Handle frame.
                 await self._handle_frame(frame)
 
@@ -951,13 +997,46 @@ class BaseOutputTransport(FrameProcessor):
                 # If we have a frame we check it's presentation timestamp. If it
                 # has already passed we process it, otherwise we wait until it's
                 # time to process it.
+                #
+                # When a TTSStoppedFrame with a pts is in flight, this queue and
+                # the audio_queue are kept in sync: the audio task signals
+                # _clock_flush_event as soon as all preceding audio has been
+                # sent, which wakes up the wait below early so every pending
+                # clock-queue frame is delivered immediately instead of stalling
+                # until its timestamp arrives.
                 if running:
                     current_time = self._transport.get_clock().get_time()
                     if timestamp > current_time:
                         wait_time = nanoseconds_to_seconds(timestamp - current_time)
-                        await asyncio.sleep(wait_time)
+                        if self._clock_flush_event:
+                            # Race between the natural timestamp and a drain
+                            # signal from the audio task. If the audio task sets
+                            # the event first, we fall through immediately so
+                            # this frame (and all subsequent ones up to the
+                            # TTSStoppedFrame) are processed without delay.
+                            try:
+                                await asyncio.wait_for(
+                                    asyncio.shield(self._clock_flush_event.wait()),
+                                    timeout=wait_time,
+                                )
+                                logger.debug(
+                                    f"{self._transport} clock queue flushed: delivering {frame} immediately"
+                                )
+                            except asyncio.TimeoutError:
+                                pass
+                        else:
+                            await asyncio.sleep(wait_time)
 
-                    # Push frame downstream.
-                    await self._transport.push_frame(frame)
+                    # If this is the TTSStoppedFrame, signal the audio task that
+                    # the drain is complete. The audio task owns the downstream
+                    # push for this frame, so skip it here.
+                    if isinstance(frame, TTSStoppedFrame) and self._clock_drained_event is not None:
+                        logger.debug(
+                            f"{self._transport} clock queue drained, handing off TTSStoppedFrame to audio queue"
+                        )
+                        self._clock_drained_event.set()
+                    else:
+                        # Push frame downstream.
+                        await self._transport.push_frame(frame)
 
                 self._clock_queue.task_done()