Batch transcription

Hopper serves Nemotron ASR, NVIDIA’s open speech-to-text model, through an OpenAI-compatible transcription endpoint. Send a complete audio file to POST https://api.withhopper.com/v1/audio/transcriptions and the response returns the transcript with word-level timestamps. For live audio, use the streaming WebSocket.

Request

The request is multipart/form-data; any other content type returns 400 invalid_content_type.

Field	Type	Constraint	Default
`file`	file	Required. Raw PCM16 mono or WAV only. Compressed formats (mp3, ogg, flac, mp4, m4a) are detected by magic bytes and rejected with 400 `unsupported_audio_format`. Total body capped at 25 MB (413 past it).	—
`model`	string	Required; 400 `missing_model` without it. Use `nemotron-asr`.	—
`language`	string	Optional language hint, forwarded to the model.	none
`keyterms`	string	Optional comma-separated list of terms to bias recognition toward (names, product terms, jargon).	none
`sample_rate`	int	Must be in the model’s allowlist — for `nemotron-asr`: 8000, 16000, 22050, 24000, 44100, 48000. Anything else returns 400 `invalid_sample_rate`.	16000

Decode compressed audio to PCM16 or WAV client-side before uploading; the gateway does not transcode.

import requests

resp = requests.post(
    "https://api.withhopper.com/v1/audio/transcriptions",
    headers={"Authorization": "Bearer sk_hopper_..."},
    files={"file": open("call.wav", "rb")},
    data={"model": "nemotron-asr", "sample_rate": 16000},
)
result = resp.json()
print(result["text"])
print(resp.headers["x-hopper-request-cost-usd"])

import { readFile } from "node:fs/promises";

const form = new FormData();
form.append("file", new Blob([await readFile("call.wav")]), "call.wav");
form.append("model", "nemotron-asr");
form.append("sample_rate", "16000");

const res = await fetch("https://api.withhopper.com/v1/audio/transcriptions", {
  method: "POST",
  headers: { Authorization: "Bearer sk_hopper_..." },
  body: form,
});
const { text, words, duration_seconds } = await res.json();

curl https://api.withhopper.com/v1/audio/transcriptions \
  -H "Authorization: Bearer sk_hopper_..." \
  -F file=@call.wav \
  -F model=nemotron-asr \
  -F sample_rate=16000

Response

{
  "text": "thanks for calling, how can I help",
  "words": [
    { "word": "thanks", "start": 0.12, "end": 0.34 }
  ],
  "duration_seconds": 12.345
}

words carries start and end offsets in seconds for every word in the finalized transcript. duration_seconds is derived from the upload, rounded to the millisecond:

duration_seconds = bytes / (sample_rate × 2)   # PCM16 mono: 2 bytes per sample

Every response includes exact spend headers: x-hopper-request-cost-usd for this request and x-hopper-credits-remaining-usd for the balance after it.

Billing

Transcription costs $0.30 per hour of audio ($0.005 per minute), metered on duration_seconds. A 10-minute call costs $0.05.

Errors

Status	Code	When
400	`invalid_content_type`	Body is not `multipart/form-data`
400	`missing_model`	No `model` field
400	`unsupported_audio_format`	Compressed audio (mp3, ogg, flac, mp4, m4a)
400	`invalid_sample_rate`	`sample_rate` not in the model’s allowlist
413	`body_too_large`	Body over 25 MB
502	`upstream_unavailable`	Transcription backend unreachable
503	`model_offline`	Model has no healthy upstream

Authentication, rate-limit (429), and credit (402) errors are shared across the API — see Errors.

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Text to Speech

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Batch transcription

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Request

Response

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Errors