> ## Documentation Index > Fetch the complete documentation index at: https://docs.withhopper.com/llms.txt > Use this file to discover all available pages before exploring further. # Transcribe audio > Batch transcription with word timestamps. Accepts raw PCM16 mono or WAV only; compressed formats (mp3, ogg, flac, mp4, m4a) are rejected. Body cap: 25 MB. ## OpenAPI ````yaml /api-reference/openapi.json post /v1/audio/transcriptions openapi: 3.1.0 info: title: Hopper API version: 1.0.0 description: >- Inference for voice agents: LLM, text to speech, and speech to text at api.withhopper.com. OpenAI-compatible routes are served under /v1; native routes are served at the root. All errors use the OpenAI envelope. servers: - url: https://api.withhopper.com security: - bearerAuth: [] tags: - name: LLM description: OpenAI-compatible chat completions and model listing. - name: Text to Speech description: >- Speech synthesis: native /tts routes and the OpenAI-compatible /v1/audio/speech. - name: Speech to Text description: Batch transcription. Streaming runs over WS /stt/websocket. - name: Voices description: Voice library, cloning, profiles, and localization. - name: Pronunciation description: Pronunciation dictionaries applied at synthesis time. - name: Jobs description: Status of async voice jobs. - name: Health description: Unauthenticated liveness check. paths: /v1/audio/transcriptions: post: tags: - Speech to Text summary: Transcribe audio description: >- Batch transcription with word timestamps. Accepts raw PCM16 mono or WAV only; compressed formats (mp3, ogg, flac, mp4, m4a) are rejected. Body cap: 25 MB. requestBody: required: true content: multipart/form-data: schema: type: object required: - file - model properties: file: type: string format: binary description: >- Raw PCM16 mono or WAV audio. Compressed formats return 400 unsupported_audio_format. model: type: string description: STT model id, e.g. nemotron-asr. language: type: string description: Language hint, forwarded to the model. keyterms: type: string description: Terms to bias recognition toward, forwarded to the model. sample_rate: type: integer enum: - 8000 - 16000 - 22050 - 24000 - 44100 - 48000 default: 16000 description: Sample rate of the submitted audio. responses: '200': description: >- Transcript with word timestamps. duration_seconds = bytes / (sample_rate x 2), rounded to ms. headers: x-hopper-request-cost-usd: $ref: '#/components/headers/XHopperRequestCostUsd' x-hopper-credits-remaining-usd: $ref: '#/components/headers/XHopperCreditsRemainingUsd' content: application/json: schema: type: object properties: text: type: string words: type: array items: type: object properties: word: type: string start: type: number description: Seconds. end: type: number description: Seconds. duration_seconds: type: number example: text: the quick brown fox words: - word: the start: 0.08 end: 0.24 - word: quick start: 0.24 end: 0.52 duration_seconds: 12.345 '400': description: >- Not multipart, missing model, compressed audio, or a sample rate outside the model's allowlist. Codes: invalid_content_type, missing_model, unsupported_audio_format, invalid_sample_rate. content: application/json: schema: $ref: '#/components/schemas/Error' '401': $ref: '#/components/responses/Unauthorized' '402': $ref: '#/components/responses/PaymentRequired' '413': description: 'Body over 25 MB. Code: body_too_large.' content: application/json: schema: $ref: '#/components/schemas/Error' '429': $ref: '#/components/responses/RateLimited' '502': description: 'Transcription upstream failed. Code: upstream_unavailable.' content: application/json: schema: $ref: '#/components/schemas/Error' '503': description: 'Model not available. Code: model_offline.' content: application/json: schema: $ref: '#/components/schemas/Error' components: headers: XHopperRequestCostUsd: description: >- Cost of this request, 6-decimal USD string (e.g. 0.000123). Absent on LLM streaming responses; the cost is unknown until the stream ends. schema: type: string XHopperCreditsRemainingUsd: description: >- Credit balance in USD, 6-decimal string. On LLM streaming responses this is the pre-request balance. schema: type: string schemas: Error: type: object description: OpenAI error envelope, used by every error response. properties: error: type: object properties: message: type: string type: type: string code: type: string param: type: - string - 'null' responses: Unauthorized: description: >- Missing, malformed, or revoked API key. Codes: invalid_api_key, api_key_disabled. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: >- Incorrect API key provided. Check your key at https://withhopper.com/dashboard. type: invalid_request_error code: invalid_api_key param: null PaymentRequired: description: Credit balance is zero. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: You have run out of credits. Contact us to add more. type: insufficient_quota code: insufficient_quota param: null RateLimited: description: >- Over 600 requests per minute on this key. Retry after the number of seconds in Retry-After. headers: Retry-After: description: Seconds to wait before retrying. schema: type: integer content: application/json: schema: $ref: '#/components/schemas/Error' example: error: message: >- Rate limit reached: 600 requests per minute per key. Retry after 21s. type: rate_limit_error code: rate_limit_exceeded param: null securitySchemes: bearerAuth: type: http scheme: bearer description: >- API key in the Authorization header: Bearer sk_hopper_... Keys are minted in the dashboard at withhopper.com/console/keys and shown once at creation. WebSocket connections also accept ?api_key=. ````