API OverviewWebSocket

Create a WebSocket session

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Create a [WebSocket session](/api/sessions) with no phone leg. The session acts as an independent participant and exchanges audio with the configured external WebSocket server. Only `type: "websocket"` is supported. The session starts in the `connecting` state. After the WebSocket connection is established, the session advances to `answered`. Its lifetime is bound to that connection. If the server disconnects or does not connect before the timeout, the session ends and a `session.ended` webhook is emitted. You can [add or remove the session from rooms](/api/room-members) and use all supported session commands. Closing the WebSocket ends the session. This endpoint creates a new session. **Triggered webhooks:** `websocket.connected`, `websocket.disconnected`, `websocket.failed`, `websocket.audio_playback_completed`, `session.ended` See [WebSocket lifecycle events](/api/web-socket#lifecycle-events) for complete payload schemas and examples. See [Session lifecycle events](/api/sessions#lifecycle-events) for complete payload schemas and examples.

Authentication

AuthorizationBearer
Application auth. Send `Authorization: Bearer <app_uuid>:<api_key>` using any active key in the app's collection. See [Authentication](https://voice-platform.docs.buildwithfern.com/api/authentication) for details.

Headers

Idempotency-KeystringOptionalformat: "^[a-zA-Z0-9._-]+$"<=128 characters
Optional key for safely retrying mutating requests. See [Idempotency](https://voice-platform.docs.buildwithfern.com/idempotency) for details.

Request

This endpoint expects an object.
typeenumRequired

Session type to create. Only websocket is supported.

Allowed values:
websocketobjectRequired

WebSocket audio connection configuration. Audio in both directions is raw mono PCM16 or G.711 mu-law, independently selected, and carried in binary WebSocket frames. Do not include a WAV header.

Response

Session created. Born in the connecting state.

created_atdatetime

When the session was created, as an RFC 3339 / ISO 8601 UTC string (e.g. “2025-05-06T12:41:36.000Z”). Storage resolution is one second, so the millisecond fraction is always .000. Empty string if the originating timestamp is missing or unparseable.

session_uuidstring
Session identifier.
stateenum

Current lifecycle state of the session. connecting applies to a websocket session whose WebSocket server connection has not yet been established.

typeenum

The nature of the session:

  • phone_in: inbound phone call from a carrier
  • phone_out: outbound phone call placed with dial
  • sip: a registered SIP device
  • webrtc: a WebRTC client
  • websocket: a WebSocket session with no phone leg
  • unknown: could not be determined
caller_idstringOptional

Caller ID in canonical international E.164 format with leading + (e.g. +972500000000). Empty for websocket sessions (no phone leg).

didstringOptional

Called DID in canonical E.164 format with leading + (e.g. +972740000000). Empty for websocket sessions (no phone leg).

Errors

400
Bad Request Error
401
Unauthorized Error
403
Forbidden Error
409
Conflict Error
413
Content Too Large Error
415
Unsupported Media Type Error
422
Unprocessable Entity Error
500
Internal Server Error
503
Service Unavailable Error