Add an opt-in streaming-audio adapter seam to BasePlatformAdapter so
voice-capable gateway platforms (LiveKit, Discord voice, future adapters)
can consume LLM output as streaming PCM audio before the full response
completes, dropping perceived voice latency from ~2-3.5s to ~500-800ms.
Adapter contract (gateway/platforms/base.py):
- AudioFormat dataclass: declared sample_rate, channels, sample_width
- StreamingTTSHandle: opaque handle with audible/aborted flags
- supports_streaming_tts / begin_streaming_tts / write_streaming_tts
/ finish_streaming_tts / abort_streaming_tts
- All default to unsupported/no-op so existing adapters are source-compatible
- Per-turn _streaming_tts_completed_chats set suppresses duplicate whole-file
auto-TTS when streaming succeeded; cleared after turn completion
Gateway consumer (gateway/streaming_tts_consumer.py):
- StreamingTTSConsumer: bridges sync agent deltas to async adapter audio sink
- Uses existing SentenceChunker (no competing parser)
- Thread-safe bounded queue; on_delta never blocks the agent worker thread
- Resolves configured streaming provider via resolve_streaming_provider()
- Serialises clause playback in order; flushes tail on completion
- Pre-audio failure: completed=False (falls back to whole-file TTS)
- Post-audio failure: completed=True, partial=True (no replay from start)
- Abort is idempotent; late chunks silently dropped
- Per-turn state isolated across concurrent chats
Gateway integration (gateway/run.py):
- message_type parameter threaded through _run_agent -> _run_agent_inner
- StreamingTTSConsumer created when voice input + auto-TTS + provider active
- Delta callback teed to both text stream consumer and TTS consumer
- TTS-only delta callback installed when text streaming is off
- finish() called from executor; wait_complete() in async context after
- Barge-in aborts the consumer at all three interrupt detection points
- Runner-level _send_voice_reply suppressed when streaming TTS completed
Tests (tests/gateway/test_streaming_tts_consumer.py):
- 15 focused tests: adapter defaults, lifecycle, ordered chunks,
unsupported/No-streamer fallback, abort idempotency, late-chunk drop,
pre/post-audio failure, concurrent-turn isolation, think-block suppression,
queue backpressure
Does not touch desktop/TUI code or add config flags. Plugin TTS provider
stream() metadata gap (#47896) is explicitly out of scope — built-in
ElevenLabs/OpenAI PCM streamers are the first consumers.
Refs: #60671, #47896
Carl Taylor