> ## Documentation Index > Fetch the complete documentation index at: https://docs.fish.audio/llms.txt > Use this file to discover all available pages before exploring further. # Realtime protocol reference > What the OpenAI-compatible Realtime WebSocket speaks: events, formats, turn detection, and transcription sessions. ``` wss://api.fish.audio/compat/v1/realtime?model=fish-audio/s2.1-pro ``` Authentication is the standard `Authorization: Bearer` header, or `?api_key=` in the URL. The OpenAI SDK derives this URL from the same client you configured for REST — a working example is in [Migrate from OpenAI](/developer-guide/compat/migrate-from-openai#realtime-websocket). ## Events The endpoint speaks the Realtime **GA** event names (`response.output_audio.delta` and the rest of the GA set) — what current `openai` packages emit, even though the Node module path still says `beta`. Supported client events: `session.update`, `transcription_session.update`, `conversation.item.create`, `conversation.item.delete`, `input_audio_buffer.append` / `commit` / `clear`, `response.create`, `response.cancel`. `conversation.item.truncate` is accepted and does nothing — it cuts a previous assistant message's audio, and none is retained server-side — so it is the one client event that sends no acknowledgement back. Any other event type comes back as an `unknown_event_type` error. One response at a time per session: a second `response.create` while one is running returns the `response_already_active` error. ## Audio formats Output formats: `pcm16`, `wav`, `mp3`, `opus` — as a string (`"mp3"`) or the GA object form (`{"type": "audio/mp3", "rate": 32000}`). `audio/pcmu` and `audio/pcma` are rejected explicitly. **Both spellings reset the rate when they carry no `rate` of their own** — to that format's default (`audio/pcm` → 24000, `wav` and `mp3` → 44100, `opus` → 48000), not to what the session was using before — so set `rate` explicitly when you switch formats mid-session. `session.updated` echoes the resulting output *and* input format — rate included — so you can read back what the session settled on. Supported sample rates per codec are in [Audio formats](/developer-guide/compat/capabilities#audio-formats). ## Voice and speed `voice` is a Fish voice ID, accepted both at the legacy top-level `session.voice` and at the GA `session.audio.output.voice` position; session events echo it at the GA position. `speed` follows the same two-position rule and the REST endpoint's 0.25–4.0 range — out of range returns an `invalid_speed` error event instead of synthesizing. ## Turn detection Turn detection is yours to drive: send `input_audio_buffer.commit` to end an utterance. Any `turn_detection` with a type — `server_vad`, `semantic_vad`, or anything else, in either the GA or the legacy top-level position — is refused with `unsupported_turn_detection` rather than accepted and ignored, and `session.created` reports `"turn_detection": null`. Likewise, an `output_modalities` that omits `"audio"` is refused: this socket always produces audio. ## Transcription Transcription works over the same socket: `input_audio_buffer.append` base64 audio, then `input_audio_buffer.commit` — the transcript arrives as `conversation.item.input_audio_transcription.completed`, and a failed transcription arrives as the matching `conversation.item.input_audio_transcription.failed` (the buffered audio is kept, so you can retry). The socket returns the transcript text only — for word timestamps use `POST /v1/audio/transcriptions`. Both official ways of opening a **dedicated** transcription session work: send `transcription_session.update`, or connect with `?intent=transcription` in the URL. Either way the session object comes back as `realtime.transcription_session` in `transcription_session.created` / `transcription_session.updated`. With `?intent=transcription` and no `model` in the URL, the session defaults to `fish-audio/transcribe-1` rather than a TTS model. ## Limits Session limits — text and audio caps per conversation item, item counts, frame size — are in the [limits table](/developer-guide/compat/capabilities#limits-and-operational-contract). ## Related A runnable Realtime example with the official SDK. The full contract: mappings, limits, and explicit refusals.