> ## Documentation Index > Fetch the complete documentation index at: https://assemblyai.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Transcribe a short audio file ## OpenAPI ````yaml specs/sync-api.yaml POST /transcribe openapi: 3.1.0 info: title: AssemblyAI Sync STT API description: Synchronous short-form audio transcription using Universal-3.5 Pro. version: 1.0.0 servers: - url: https://sync.assemblyai.com description: Global (default — routes to nearest region) - url: https://sync.us.assemblyai.com description: US data residency (us-west-2, us-east-1) - url: https://sync.eu.assemblyai.com description: EU data residency (eu-north-1) security: - ApiKey: [] paths: /transcribe: post: summary: Transcribe a short audio file description: > Submit an audio file and receive the transcript synchronously in a single request/response. No polling required. Audio must be between 80 ms and 120 s. To use a data residency endpoint, replace `sync.assemblyai.com` with `sync.us.assemblyai.com` (US) or `sync.eu.assemblyai.com` (EU). See [Cloud Endpoints and Data Residency](/sync-stt/endpoints-and-data-zones) for more information. operationId: transcribeSync parameters: - in: header name: X-AAI-Model required: true schema: type: string enum: - universal-3-5-pro description: >- Model identifier used for request routing. The canonical value is `universal-3-5-pro`; `u3-sync-pro` and `u3-pro` are accepted as legacy aliases. - in: query name: token required: false schema: type: string description: >- API key alternative for clients that cannot set the `Authorization` header. The header wins when both are present. requestBody: required: true content: multipart/form-data: schema: type: object required: - audio properties: audio: type: string format: binary description: >- Raw audio bytes. Set the part's Content-Type to `audio/wav` for WAV files or `audio/pcm` for raw S16LE little-endian PCM. config: type: object description: Optional transcription configuration. properties: sample_rate: type: integer enum: - 8000 - 16000 - 22050 - 24000 - 32000 - 44100 - 48000 description: >- Source sample rate in Hz. Required when `audio` is `audio/pcm`. WAV reads the rate from its header. channels: type: integer enum: - 1 - 2 description: >- Number of audio channels. Required when `audio` is `audio/pcm`. Stereo is down-mixed to mono internally. prompt: type: string maxLength: 4096 description: >- Custom transcription instruction prepended to the model's system prompt. When omitted, a default prompt is applied. keyterms_prompt: type: array items: type: string description: >- Keyterms that bias the decoder toward specific tokens. Maximum 2048 characters total across all terms. Also accepted as `keyterms` or `word_boost` — provide only one of the three. conversation_context: oneOf: - type: string - type: array items: type: string description: >- Prior turns from the same conversation, in chronological order (oldest first, most recent last). Supplies the preceding dialogue as context so the model transcribes the current clip with greater continuity across a multi-turn conversation (for example, a human speaking with a voice agent). Accepts either a list of turns (for multi-turn conversations) or a single string (treated as one turn). If the number of turns exceeds the context window limit, the oldest turns are dropped first. language_code: default: en description: >- Language of the audio as an ISO 639-1 code, or a list of codes for multilingual audio. Steers the default transcription prompt toward the named language(s). Ignored when a custom `prompt` is set. Defaults to `en`. oneOf: - type: string enum: - en - es - de - fr - it - pt - tr - nl - sv - 'no' - da - fi - hi - vi - ar - he - ja - ur - zh - type: array items: type: string enum: - en - es - de - fr - it - pt - tr - nl - sv - 'no' - da - fi - hi - vi - ar - he - ja - ur - zh encoding: audio: contentType: audio/wav, audio/pcm config: contentType: application/json responses: '200': description: Transcription completed successfully. content: application/json: schema: $ref: '#/components/schemas/SyncTranscriptResponse' example: text: Hi, I'm calling about my Best Buy order... words: - text: Hi start: 0 end: 200 confidence: 0.91 - text: I'm start: 220 end: 320 confidence: 0.88 confidence: 0.87 audio_duration_ms: 101567 session_id: eb92c4ff-4bbb-429f-9b99-7279d7fe738f '400': description: Bad request — audio too short, malformed, or invalid config. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '401': description: Missing or invalid API key. content: application/json: schema: $ref: '#/components/schemas/DetailErrorResponse' '413': description: Audio exceeds the 120 s duration or 40 MB size limit. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '415': description: Unsupported media type or audio format. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '429': description: Rate limit exceeded. Retry after `Retry-After` seconds. content: application/json: schema: $ref: '#/components/schemas/DetailErrorResponse' '500': description: Internal model error. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '503': description: >- Service unavailable — model cold-starting or concurrency cap reached. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '504': description: Request exceeded the 30 s per-request deadline. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' components: schemas: SyncTranscriptResponse: type: object required: - text - words - confidence - audio_duration_ms - session_id properties: text: type: string description: Full transcript of the audio. words: type: array items: $ref: '#/components/schemas/Word' description: Word-level timestamps and confidence scores. confidence: type: number format: float description: Overall transcript confidence (0–1). audio_duration_ms: type: integer description: Duration of the submitted audio in milliseconds. session_id: type: string format: uuid description: >- Server-generated request identifier. Include this in support requests. ErrorResponse: type: object properties: error_code: type: string description: Machine-readable error code. enum: - bad_audio - audio_too_short - audio_too_large - bad_request - unsupported_media_type - capacity_exceeded - service_unavailable - inference_timeout - inference_error message: type: string description: Human-readable error description. DetailErrorResponse: type: object properties: detail: type: string description: >- Human-readable error description (used for auth and rate-limit errors). Word: type: object required: - text - start - end - confidence properties: text: type: string start: type: integer description: Word start time in milliseconds. end: type: integer description: Word end time in milliseconds. confidence: type: number format: float securitySchemes: ApiKey: type: apiKey in: header name: Authorization description: >- Your AssemblyAI API key. Optionally prefixed with `Bearer `. Also accepted as the `token` query parameter. ````