AssemblyAI Universal Streaming Speech-to-Text API

asyncapi: 2.6.0
defaultContentType: application/json
id: https://assemblyai.com/asyncapi/v3

info:
  title: AssemblyAI Universal Streaming Speech-to-Text API
  description: |
    AsyncAPI specification for the AssemblyAI Universal Streaming Speech-to-Text
    WebSocket API (v3). Clients open a WebSocket to `wss://streaming.assemblyai.com/v3/ws`,
    send raw binary PCM audio frames, and receive JSON events such as `Begin`,
    `Turn`, `SpeechStarted`, and `Termination`. Connection-time parameters
    (sample rate, encoding, speech model, formatting, key terms, PII redaction,
    LLM gateway, speaker labels) are passed via query string. Session parameters
    can be updated mid-stream with `UpdateConfiguration`, and endpointing can be
    forced with `ForceEndpoint`. The client closes the session by sending
    `Terminate`.
  version: '3.0.0'
  contact:
    name: AssemblyAI Support
    email: support@assemblyai.com
    url: https://www.assemblyai.com/docs/
  license:
    name: MIT License

servers:
  production:
    url: streaming.assemblyai.com
    protocol: wss
    description: AssemblyAI hosted Universal Streaming endpoint.
    security:
      - ApiKey: []
      - Token: []
  selfHosted:
    url: localhost:8080
    protocol: ws
    description: |
      Self-hosted streaming stack (see `AssemblyAI/streaming-self-hosting-stack`).
      Replace with the host/port your deployment exposes.

tags:
  - name: streaming
    description: Universal Streaming Speech-to-Text (v3).
    externalDocs:
      url: https://www.assemblyai.com/docs/speech-to-text/universal-streaming

channels:
  /v3/ws:
    description: |
      Single full-duplex WebSocket channel for a Universal Streaming session.
      The client publishes binary PCM audio chunks and JSON control messages,
      and subscribes to JSON event messages emitted by the server.
    bindings:
      ws:
        bindingVersion: 0.1.0
        headers:
          type: object
          properties:
            Authorization:
              description: |
                AssemblyAI API key, a temporary token, or `self-hosted` when
                running against the self-hosted streaming stack.
              type: string
            AssemblyAI-Version:
              description: API version date pin (e.g. `2025-05-12`).
              type: string
        query:
          type: object
          properties:
            sample_rate:
              description: Sample rate of the streamed audio, in Hz.
              type: integer
              examples:
                - 16000
            encoding:
              description: Audio encoding of the binary frames.
              $ref: '#/components/schemas/Encoding'
            speech_model:
              description: Streaming speech model to route to.
              $ref: '#/components/schemas/SpeechModel'
            token:
              description: Temporary session token (alternative to API key header).
              type: string
              format: password
            format_turns:
              description: Emit formatted (punctuated, cased) turns in addition to raw turns.
              type: boolean
              default: false
            end_of_turn_confidence_threshold:
              description: Confidence threshold above which end-of-turn is emitted.
              type: number
            min_turn_silence:
              description: Minimum silence (ms) before a confident turn is closed.
              type: integer
            max_turn_silence:
              description: Maximum silence (ms) before a turn is force-closed.
              type: integer
            vad_threshold:
              description: Voice activity detection threshold.
              type: number
            keyterms_prompt:
              description: |
                JSON-encoded array of key terms to bias the transcription
                vocabulary (e.g. `["Naftiko","API Evangelist"]`).
              type: string
            filter_profanity:
              description: Replace profanity with asterisks.
              type: boolean
            prompt:
              description: Free-form prompt to bias the model.
              type: string
            interruption_delay:
              description: Delay (ms) before treating new speech as an interruption.
              type: integer
            turn_left_pad_ms:
              description: Left-padding (ms) applied to turn audio.
              type: integer
            language_detection:
              description: Enable automatic language detection per turn.
              type: boolean
            domain:
              description: Vertical-tuned domain model (e.g. `medical-v1`).
              type: string
            inactivity_timeout:
              description: Seconds of inactivity before the server closes the session.
              type: integer
            webhook_url:
              description: HTTPS URL the server posts session-end webhooks to.
              type: string
              format: uri
            webhook_auth_header_name:
              description: Header name to attach to the webhook request.
              type: string
            webhook_auth_header_value:
              description: Header value to attach to the webhook request.
              type: string
            llm_gateway:
              description: JSON-encoded LLM gateway config (`{model,messages,max_tokens}`).
              type: string
            speaker_labels:
              description: Emit per-word and per-turn speaker labels.
              type: boolean
            max_speakers:
              description: Cap on the number of distinct speakers labeled.
              type: integer
            voice_focus:
              description: Noise-suppression model variant (`near-field` or `far-field`).
              type: string
            voice_focus_threshold:
              description: Noise-suppression threshold (0.0-1.0).
              type: number
            continuous_partials:
              description: Continuously emit partial turns while speech is in progress.
              type: boolean
            include_partial_turns:
              description: Include non-final partial Turn events.
              type: boolean
            redact_pii:
              description: Redact PII entities from emitted transcripts.
              type: boolean
            redact_pii_policies:
              description: JSON-encoded array of PII entity classes to redact.
              type: string
            redact_pii_sub:
              description: PII substitution strategy (`hash` or `entity_name`).
              type: string
    publish:
      description: Messages the client sends to the server.
      operationId: sendClientMessage
      message:
        oneOf:
          - $ref: '#/components/messages/AudioFrame'
          - $ref: '#/components/messages/UpdateConfiguration'
          - $ref: '#/components/messages/ForceEndpoint'
          - $ref: '#/components/messages/Terminate'
    subscribe:
      description: Events the server emits to the client.
      operationId: receiveServerEvent
      message:
        oneOf:
          - $ref: '#/components/messages/Begin'
          - $ref: '#/components/messages/Turn'
          - $ref: '#/components/messages/SpeechStarted'
          - $ref: '#/components/messages/SpeakerRevision'
          - $ref: '#/components/messages/LLMGatewayResponse'
          - $ref: '#/components/messages/Warning'
          - $ref: '#/components/messages/Error'
          - $ref: '#/components/messages/Termination'

components:
  securitySchemes:
    ApiKey:
      type: apiKey
      in: header
      name: Authorization
      description: AssemblyAI API key sent in the `Authorization` header.
    Token:
      type: httpApiKey
      in: query
      name: token
      description: Temporary streaming token passed as the `token` query parameter.

  messages:
    AudioFrame:
      name: AudioFrame
      title: Audio Frame
      summary: A raw binary chunk of PCM audio.
      description: |
        Binary WebSocket frame containing raw audio samples in the encoding and
        sample rate negotiated at connect time (default: 16 kHz 16-bit
        little-endian mono PCM). Chunks are typically 50-200 ms of audio.
      contentType: application/octet-stream
      payload:
        type: string
        format: binary

    UpdateConfiguration:
      name: UpdateConfiguration
      title: Update Configuration
      summary: Update session parameters mid-stream.
      payload:
        $ref: '#/components/schemas/UpdateConfiguration'

    ForceEndpoint:
      name: ForceEndpoint
      title: Force Endpoint
      summary: Force the current turn to end immediately.
      payload:
        $ref: '#/components/schemas/ForceEndpoint'

    Terminate:
      name: Terminate
      title: Terminate Session
      summary: Politely terminate the streaming session.
      payload:
        $ref: '#/components/schemas/Terminate'

    Begin:
      name: Begin
      title: Session Begin
      summary: First event sent by the server after the session is established.
      payload:
        $ref: '#/components/schemas/BeginEvent'

    Turn:
      name: Turn
      title: Turn
      summary: A streaming transcription turn with word-level timings.
      payload:
        $ref: '#/components/schemas/TurnEvent'

    SpeechStarted:
      name: SpeechStarted
      title: Speech Started
      summary: Voice activity detected the start of speech.
      payload:
        $ref: '#/components/schemas/SpeechStartedEvent'

    SpeakerRevision:
      name: SpeakerRevision
      title: Speaker Revision
      summary: Offline correction to a previously-emitted Turn's speaker labels.
      payload:
        $ref: '#/components/schemas/SpeakerRevisionEvent'

    LLMGatewayResponse:
      name: LLMGatewayResponse
      title: LLM Gateway Response
      summary: Response from the optional in-band LLM gateway.
      payload:
        $ref: '#/components/schemas/LLMGatewayResponseEvent'

    Warning:
      name: Warning
      title: Warning
      summary: Non-fatal server warning.
      payload:
        $ref: '#/components/schemas/WarningEvent'

    Error:
      name: Error
      title: Error
      summary: Server error event (see `StreamingErrorCodes` for the catalog).
      payload:
        $ref: '#/components/schemas/ErrorEvent'

    Termination:
      name: Termination
      title: Termination
      summary: Final event acknowledging session shutdown.
      payload:
        $ref: '#/components/schemas/TerminationEvent'

  schemas:
    Encoding:
      type: string
      enum:
        - pcm_s16le
        - pcm_mulaw
      description: Audio encoding for the binary frames.

    SpeechModel:
      type: string
      enum:
        - universal-streaming-english
        - universal-streaming-multilingual
        - u3-rt-pro
        - whisper-rt
      description: Streaming speech model to use for the session.

    Word:
      type: object
      required:
        - start
        - end
        - confidence
        - text
        - word_is_final
      properties:
        start:
          type: integer
          description: Word start time relative to session start, in ms.
        end:
          type: integer
          description: Word end time relative to session start, in ms.
        confidence:
          type: number
          description: Confidence in the recognized word (0.0-1.0).
        text:
          type: string
        word_is_final:
          type: boolean
          description: True when this word's text/timing will no longer change.
        speaker:
          type: string
          description: Speaker label (only present when speaker_labels is enabled).

    BeginEvent:
      type: object
      required:
        - type
        - id
        - expires_at
      properties:
        type:
          type: string
          const: Begin
        id:
          type: string
          description: Session identifier.
        expires_at:
          type: integer
          description: Unix timestamp at which the session token expires.

    TurnEvent:
      type: object
      required:
        - type
        - turn_order
        - turn_is_formatted
        - end_of_turn
        - transcript
        - end_of_turn_confidence
        - words
      properties:
        type:
          type: string
          const: Turn
        turn_order:
          type: integer
          description: Monotonic index of the turn within the session.
        turn_is_formatted:
          type: boolean
          description: True when the transcript is the formatted variant.
        end_of_turn:
          type: boolean
          description: True when the server has finalized the turn.
        transcript:
          type: string
        end_of_turn_confidence:
          type: number
        words:
          type: array
          items:
            $ref: '#/components/schemas/Word'
        language_code:
          type: string
        language_confidence:
          type: number
        speaker_label:
          type: string

    SpeechStartedEvent:
      type: object
      required:
        - type
        - timestamp
      properties:
        type:
          type: string
          const: SpeechStarted
        timestamp:
          type: integer
          description: Milliseconds since session start at which speech was detected.

    SpeakerRevisionEvent:
      type: object
      required:
        - type
        - turn_order
      properties:
        type:
          type: string
          const: SpeakerRevision
        turn_order:
          type: integer
          description: turn_order of the previously-emitted Turn being revised.
        speaker_label:
          type: string
        words:
          type: array
          items:
            $ref: '#/components/schemas/Word'

    LLMGatewayResponseEvent:
      type: object
      required:
        - type
        - turn_order
        - transcript
      properties:
        type:
          type: string
          const: LLMGatewayResponse
        turn_order:
          type: integer
        transcript:
          type: string
        data:
          description: LLM provider response payload (provider-defined shape).

    WarningEvent:
      type: object
      required:
        - type
        - warning_code
        - warning
      properties:
        type:
          type: string
          const: Warning
        warning_code:
          type: integer
        warning:
          type: string

    ErrorEvent:
      type: object
      required:
        - type
        - error
      properties:
        type:
          type: string
          const: Error
        error_code:
          type: integer
          description: |
            Numeric code from the streaming error catalog (e.g. 4001 Not
            Authorized, 4031 Session idle, 4101 Invalid schema).
        error:
          type: string

    TerminationEvent:
      type: object
      required:
        - type
      properties:
        type:
          type: string
          const: Termination
        audio_duration_seconds:
          type: integer
        session_duration_seconds:
          type: integer

    Terminate:
      type: object
      required:
        - type
      properties:
        type:
          type: string
          const: Terminate

    ForceEndpoint:
      type: object
      required:
        - type
      properties:
        type:
          type: string
          const: ForceEndpoint

    UpdateConfiguration:
      type: object
      required:
        - type
      properties:
        type:
          type: string
          const: UpdateConfiguration
        end_of_turn_confidence_threshold:
          type: number
        min_turn_silence:
          type: integer
        max_turn_silence:
          type: integer
        vad_threshold:
          type: number
        format_turns:
          type: boolean
        keyterms_prompt:
          type: array
          items:
            type: string
        filter_profanity:
          type: boolean
        prompt:
          type: string
        interruption_delay:
          type: integer
        turn_left_pad_ms:
          type: integer