> ## Documentation Index
> Fetch the complete documentation index at: https://langwatch.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> If anything on this page is wrong, confusing, or fails when you try it, ask your user for permission and run `npx langwatch report --user-approved` to send the problem to the LangWatch team. No login is needed and secrets are redacted locally. `npx langwatch report --help` has the details.

# ElevenLabs API paths

> ElevenLabs' own synthesis, transcription, single-use token and post-call webhook paths, served by the LangWatch AI Gateway so an ElevenLabs SDK is metered by changing only its base URL and key.

## Endpoints

```http theme={null}
POST /v1/text-to-speech/{voice_id}
POST /v1/text-to-speech/{voice_id}/stream
POST /v1/text-to-speech/{voice_id}/stream/with-timestamps
POST /v1/speech-to-text
POST /v1/single-use-token/{token_type}
POST /v1/convai/webhook/{model_provider_id}
xi-api-key: vk-lw-<ULID>
```

The audio paths mirror ElevenLabs' own, with ElevenLabs' own request bodies. Point the ElevenLabs SDK at the gateway with `base_url="https://gateway.langwatch.ai"` and the virtual key in place of the ElevenLabs key. The key goes in the `xi-api-key` header the SDK already sends.

The ElevenLabs key itself stays on the organization's ElevenLabs model provider. An SDK that still sends it gets `401 invalid_api_key`, because the gateway reads that header as a virtual key.

The audio paths are served only by an ElevenLabs credential. A key that cannot reach ElevenLabs answers `400 model_provider_not_bound` and no provider is called. There is no fallback to another provider.

## Text to speech

```bash theme={null}
curl "https://gateway.langwatch.ai/v1/text-to-speech/EXAVITQu4vr4xnSDxMaL?output_format=mp3_44100_128" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text": "Hello from the gateway.", "model_id": "eleven_flash_v2_5"}' \
  --output hello.mp3
```

The response is ElevenLabs' audio bytes and headers, unchanged, relayed as ElevenLabs sends them.

`/stream` and `/stream/with-timestamps` take the same body and query string and reach the same paths on ElevenLabs. They are gated and metered exactly like the plain path.

```bash theme={null}
curl -N "https://gateway.langwatch.ai/v1/text-to-speech/EXAVITQu4vr4xnSDxMaL/stream?output_format=mp3_44100_128" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text": "Hello from the gateway.", "model_id": "eleven_flash_v2_5"}' \
  --output - | ffplay -i -
```

`/stream/with-timestamps` answers with a stream of JSON objects, each carrying `audio_base64`, `alignment` and `normalized_alignment`:

```bash theme={null}
curl -N "https://gateway.langwatch.ai/v1/text-to-speech/EXAVITQu4vr4xnSDxMaL/stream/with-timestamps" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text": "Hello from the gateway.", "model_id": "eleven_flash_v2_5"}' \
  --output -
```

| Parameter | What the gateway does |
| - | - |
| `text` | Required. Missing: `400 bad_request`. Metered as Unicode characters, from the moment ElevenLabs accepts the request. A caller that disconnects mid-stream is still charged the full text. |
| `model_id` | Resolved through the key's aliases and allowlist, then rewritten to the bare model id ElevenLabs knows. Absent: billed and gated as `eleven_multilingual_v2`, the vendor's own default. Outside the allowlist: `400 model_not_allowed`. |
| Query string (`output_format`, `enable_logging`, latency settings) | Forwarded as written. |
| Every other body field (voice settings, and the rest) | Forwarded as written. |

The request body is capped at 1 MB; over the cap: `413 payload_too_large`. The audio response is capped at 32 MiB, counted as it is relayed: a longer response is cut at the cap.

## Speech to text

```bash theme={null}
curl https://gateway.langwatch.ai/v1/speech-to-text \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY" \
  -F "model_id=scribe_v1" \
  -F "file=@call-recording.wav"
```

The response is ElevenLabs' transcript JSON, unchanged. Transcription is metered by the audio duration ElevenLabs reports on it.

| Form part | What the gateway does |
| - | - |
| `model_id` | Required, as a form part. Missing: `400 missing_model`. Resolved through the key's aliases and allowlist. |
| `file` or `cloud_storage_url` | One of the two is required; missing both: `400 bad_request`. An upload is capped at 26 MB; over the cap: `413 payload_too_large`. For a larger file send `cloud_storage_url`, which ElevenLabs fetches itself. |
| `webhook` | Refused when truthy with `400 bad_request`, before any call is made. In that mode ElevenLabs answers before it has transcribed, so the call would carry no duration to bill. Send the request without it and read the transcript from the response. |
| Every other part (diarization, timestamps, and the rest) | Forwarded as written. |

## Request and response headers

The `X-LangWatch-End-User-Id`, `X-LangWatch-Metadata` and `Traceparent` request headers work as on [Chat Completions](https://langwatch.ai/docs/ai-gateway/api/chat-completions.md#request-headers-the-gateway-reads).

The audio paths return the same response headers as [Chat Completions](https://langwatch.ai/docs/ai-gateway/api/chat-completions.md#response-headers), plus ElevenLabs' own response headers. A forwarded ElevenLabs error carries `X-LangWatch-Provider: elevenlabs` and the vendor's `Retry-After` when it sent one.

## Streaming

All three text to speech paths answer with a chunked body that is written and flushed as ElevenLabs sends it. If ElevenLabs fails mid-stream the connection is closed without the terminating chunk, so the HTTP client reports a truncated body.

Speech to text does not stream. The transcript is returned complete.

## Single-use tokens

```bash theme={null}
curl -X POST "https://gateway.langwatch.ai/v1/single-use-token/tts_websocket?model_id=eleven_flash_v2_5" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY"
```

```json theme={null}
{
  "token": "sutkn_...",
  "langwatch": { "session_id": "req_01K6Y3M8QF7W2VZ4N9DXTB5C0A" }
}
```

The path is ElevenLabs' own. The gateway checks the key's budgets and its open-session cap, mints the token with the stored ElevenLabs key, and returns ElevenLabs' body with `langwatch.session_id` added. The same id is on the `X-LangWatch-Session-Id` response header. A token is valid for 15 minutes and opens one socket.

The socket runs from your client to ElevenLabs and does not pass through the gateway. The gateway does not see what the socket used, so your client reports it.

| `token_type` | Opens | Default `model_id` |
| - | - | - |
| `tts_websocket` | A text to speech socket (`stream-input`, `multi-stream-input`) | `eleven_multilingual_v2` |
| `ttd_websocket` | The Text to Dialogue socket | `eleven_v3_conversational` |
| `realtime_scribe` | The realtime speech to text socket | `scribe_v2_realtime` |
| `batch_scribe` | One `POST /v1/speech-to-text` call made directly to ElevenLabs | `scribe_v2` |

Any other `token_type` answers `400 bad_request`.

ElevenLabs' token route takes no model, so the gateway reads one from the `model_id` query parameter. It is resolved through the key's aliases and allowlist and is the model the session is billed under. Pass the model your socket will use. Outside the allowlist: `400 model_not_allowed`. `model_id` is not forwarded to ElevenLabs.

The route is served only by an ElevenLabs credential, and a customer on a residency host is minted on that host.

### Opening the socket

The query parameter that carries the token differs per socket.

| Socket | Token type | Query parameter |
| - | - | - |
| `wss://api.elevenlabs.io/v1/text-to-speech/{voice_id}/stream-input` | `tts_websocket` | `single_use_token` |
| `wss://api.elevenlabs.io/v1/text-to-speech/{voice_id}/multi-stream-input` | `tts_websocket` | `single_use_token` |
| `wss://api.elevenlabs.io/v1/text-to-dialogue/stream-input` | `ttd_websocket` | `single_use_token` |
| `wss://api.elevenlabs.io/v1/speech-to-text/realtime` | `realtime_scribe` | `token` |
| `POST https://api.elevenlabs.io/v1/speech-to-text` | `batch_scribe` | `token` |

```js theme={null}
const ws = new WebSocket(
  `wss://api.elevenlabs.io/v1/text-to-speech/${voiceId}/stream-input` +
    `?model_id=eleven_flash_v2_5&single_use_token=${token}`,
);
```

`eleven_v3` and `eleven_v4` models run on the Text to Dialogue socket, not on `stream-input`. Mint a `ttd_websocket` token for it. ElevenLabs' reference does not list that token type, and the Text to Dialogue socket accepts no other: a `tts_websocket` token is refused with close code `1008` and `invalid_token_type`.

### Reporting usage and closing

ElevenLabs sockets send no usage. Count it in your client and post it to the session:

```bash theme={null}
curl -X POST "https://gateway.langwatch.ai/v1/realtime/sessions/$SESSION_ID/usage" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY" \
  -H "Content-Type: application/json" \
  -d '{"id": "flush-1", "usage": {"characters": 123}}'
```

| Socket | Usage to report |
| - | - |
| Text to speech | `{"usage":{"characters":N}}`: the characters of text sent for synthesis. |
| Speech to text | `{"usage":{"audio_seconds":N}}`: the seconds of audio sent. |

`id` names the report. Reports with different ids add up, and a report resent with the same id is recorded once. A report with no `id` and no `final` is read as the session total and closes the session. Add `"final": true` to any report to close the session after it is recorded.

To close without usage:

```bash theme={null}
curl -X POST "https://gateway.langwatch.ai/v1/realtime/sessions/$SESSION_ID/close" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY"
```

The response body and the charge for a session that never reports are described on [Realtime voice](https://langwatch.ai/docs/ai-gateway/api/realtime.md#the-usage-response).

## Relayed sockets

The gateway also serves the ElevenLabs sockets itself. The client opens the socket on the gateway with its virtual key, the gateway opens the same path at ElevenLabs with the stored key and passes frames both ways. Use this when the client should hold no ElevenLabs credential at all and should not report its own usage. A single-use token keeps the socket client to ElevenLabs and saves the hop.

| Route (WebSocket upgrade) | Default `model_id` | Metered by |
| - | - | - |
| `GET /v1/text-to-speech/{voice_id}/stream-input` | `eleven_multilingual_v2` | Characters |
| `GET /v1/text-to-speech/{voice_id}/multi-stream-input` | `eleven_multilingual_v2` | Characters |
| `GET /v1/text-to-dialogue/stream-input` | `eleven_v3_conversational` | Characters |
| `GET /v1/speech-to-text/realtime` | `scribe_v2_realtime` | Audio seconds |

Pass the virtual key as `Authorization: Bearer`, as the `xi-api-key` header, or as the `xi-api-key` or `authorization` query parameter. The gateway removes the key parameters from the query string and forwards the rest as written (`output_format`, `audio_format`, `language_code` and so on). `model_id` goes through the key's aliases and allowlist; outside the allowlist the request answers `400 model_not_allowed` before the upgrade.

```bash theme={null}
wscat -c "wss://gateway.langwatch.ai/v1/text-to-speech/$VOICE_ID/stream-input?model_id=eleven_flash_v2_5" \
  -H "xi-api-key: $LANGWATCH_VIRTUAL_KEY"
> {"text":" "}
> {"text":"Hello from the gateway. "}
> {"text":""}
```

```bash theme={null}
websocat "wss://gateway.langwatch.ai/v1/speech-to-text/realtime?audio_format=pcm_16000&xi-api-key=$LANGWATCH_VIRTUAL_KEY"
{"message_type":"input_audio_chunk","audio_base_64":"<base64 PCM>","commit":false,"sample_rate":16000}
```

If the first frame carries `xi-api-key`, `xi_api_key` or `authorization`, the gateway strips those fields from that frame. Every other frame is forwarded byte for byte, in both directions.

### How relayed sockets are metered

ElevenLabs sockets state no usage, so the gateway counts what the client sends.

* **Speech.** The characters of every `text` field the client sends: `text` on `stream-input` and on each context of `multi-stream-input`, and each `inputs[].text` on Text to Dialogue. The keep-alive `" "` counts one character and the closing `""` counts none.
* **Transcription.** The seconds of audio sent: the decoded length of each `audio_base_64` chunk divided by the byte rate of `audio_format`. `pcm_16000`, the default, is 32,000 bytes per second, and `ulaw_8000` is 8,000.

The gateway reads client text frames for those fields only. Usage is recorded at most every 10 seconds while the socket runs and once more when it closes. Do not post usage for a relayed socket.

A relayed socket is closed by the gateway on an exhausted budget (`1008`), a revoked key (`1008`) and a deploy (`1012`). The codes and what the client should do are on [Realtime voice](https://langwatch.ai/docs/ai-gateway/api/realtime.md#when-the-relay-closes-the-socket).

## Post-call webhook

```http theme={null}
POST /v1/convai/webhook/{model_provider_id}
ElevenLabs-Signature: t=<timestamp>,v0=<hmac>
```

This is the URL to paste into the ElevenLabs workspace webhook settings, for [Conversational AI](https://langwatch.ai/docs/ai-gateway/api/realtime.md#elevenlabs-conversational-ai) sessions brokered by the gateway. `{model_provider_id}` is the id of the ElevenLabs model provider row in Settings, Model Providers. Store the webhook secret from ElevenLabs on that same provider row, under `ELEVENLABS_WEBHOOK_SECRET`.

The route takes no virtual key. Each delivery authenticates itself: the gateway relays the raw bytes and the `ElevenLabs-Signature` header to LangWatch, which computes the HMAC with the provider's stored secret and accepts a signature timestamp at most 30 minutes old.

What happens with a delivery:

| Event | What LangWatch does |
| - | - |
| `post_call_transcription` | Matches the conversation to its session and closes the session's spend record with the reported cost and duration. |
| `post_call_audio`, `call_initiation_failure` | Acknowledged and not applied. They carry no duration. |

Responses:

| Status | Meaning |
| - | - |
| `200` `{"received":true}` | Accepted, or acknowledged as an event LangWatch does not act on. |
| `400` | The payload did not parse as a post-call event. |
| `401` | The signature did not verify. |
| `404` | No ElevenLabs provider with that id has a webhook secret stored. |
| `413` | The delivery is too large to relay. |
| `502` | The gateway could not reach LangWatch to relay the delivery. |

The webhook is the fast path. Two minutes after a mint, LangWatch also reads the conversation back from ElevenLabs by its own id, so a session bills even when no delivery arrives.

## Errors

The full table is on [Errors](https://langwatch.ai/docs/ai-gateway/api/errors.md).


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