OpenRouter Speech-to-Text

Transcribe audio via POST /api/v1/audio/transcriptions using curl. Requires OPENROUTER_API_KEY (get one at https://openrouter.ai/keys). If unset, stop and ask.

This endpoint is not OpenAI-compatible. The body is JSON with base64 audio under input_audio: { data, format } — not multipart/form-data with a file field the way OpenAI's /v1/audio/transcriptions works. Do not point the OpenAI SDK at this endpoint; it will send the wrong shape. Use curl, fetch, or requests directly.

One call, JSON back

Both request and response are JSON. The response body carries:

• text — the transcript.
• usage — always includes cost. Providers additionally report either seconds of audio billed or a token breakdown (total_tokens, input_tokens, output_tokens), depending on how they price the request. Don't assume both are present.

Sample response (duration-priced provider, e.g. google/chirp-3):

{
  "text": "I used to rule the world.",
  "usage": {
    "seconds": 20,
    "cost": 0.005333
  }
}

Sample response (token-priced provider):

{
  "text": "Hello, this is a test of speech-to-text transcription.",
  "usage": {
    "total_tokens": 113,
    "input_tokens": 83,
    "output_tokens": 30,
    "cost": 0.000508
  }
}

Drop-in workflow

#!/usr/bin/env bash
set -euo pipefail

MODEL="google/chirp-3"
FORMAT="wav"                          # wav, mp3, flac, m4a, ogg, webm, aac
AUDIO="audio.wav"
BODY=$(mktemp)
PAYLOAD=$(mktemp)

audio_b64=$(base64 < "$AUDIO" | tr -d '\n')

jq -n --arg model "$MODEL" --arg data "$audio_b64" --arg fmt "$FORMAT" \
  '{model: $model, input_audio: {data: $data, format: $fmt}}' > "$PAYLOAD"

# --data-binary @file keeps the base64 payload off argv (avoids E2BIG / ARG_MAX).
http_code=$(curl -sS -X POST https://openrouter.ai/api/v1/audio/transcriptions \
  -H "Authorization: Bearer $OPENROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  --output "$BODY" \
  -w '%{http_code}' \
  --data-binary @"$PAYLOAD")

if [[ "$http_code" != "200" ]]; then
  echo "STT failed (HTTP $http_code):" >&2
  cat "$BODY" >&2
  rm -f "$BODY" "$PAYLOAD"
  exit 1
fi

jq -r '.text' "$BODY"
rm -f "$BODY" "$PAYLOAD"

Discovering STT models

Filter the models endpoint by output modality to list transcription models.

curl -sS "https://openrouter.ai/api/v1/models?output_modalities=transcription" \
  | jq '.data[] | {id, name, pricing}'

Models are provider-namespaced — use the full slug (google/chirp-3, openai/whisper-1, openai/whisper-large-v3), not the short name.

Parameters

Picking an audio format

• wav / flac — uncompressed or lossless. Highest quality; largest uploads.
• mp3 / m4a / aac — compressed. Smaller payloads, which matters because base64 inflates bytes by ~33% on top of whatever the file already weighs.
• webm / ogg — typical for browser recordings (MediaRecorder).

The format field must match the actual container/codec of the bytes. A file saved as .wav that is actually mp3 will be rejected or mis-decoded. When in doubt, confirm with ffprobe <file>.

Provider-specific options

Provider passthrough goes under provider.options.<slug> and is only forwarded when that provider handles the request. Example — Groq's prompt for vocabulary hinting:

{
  "model": "openai/whisper-large-v3",
  "input_audio": { "data": "UklGRiQA...", "format": "wav" },
  "provider": {
    "options": {
      "groq": {
        "prompt": "Expected vocabulary: OpenRouter, API, transcription"
      }
    }
  }
}

Options keyed by provider slug are forwarded only when that provider matches; other keys are ignored. Check each provider's upstream docs for available passthrough keys.

TypeScript (fetch)

import fs from "fs";

const audio = await fs.promises.readFile("audio.wav");
const data = audio.toString("base64");

const res = await fetch("https://openrouter.ai/api/v1/audio/transcriptions", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.OPENROUTER_API_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    model: "google/chirp-3",
    input_audio: { data, format: "wav" },
  }),
});

if (!res.ok) {
  throw new Error(`STT failed (HTTP ${res.status}): ${await res.text()}`);
}

const result = await res.json();
console.log(result.text);

Python (requests)

import base64
import os
import requests

with open("audio.wav", "rb") as f:
    data = base64.b64encode(f.read()).decode("utf-8")

res = requests.post(
    "https://openrouter.ai/api/v1/audio/transcriptions",
    headers={
        "Authorization": f"Bearer {os.environ['OPENROUTER_API_KEY']}",
        "Content-Type": "application/json",
    },
    json={
        "model": "google/chirp-3",
        "input_audio": {"data": data, "format": "wav"},
    },
)

if not res.ok:
    raise RuntimeError(f"STT failed (HTTP {res.status_code}): {res.text}")

print(res.json()["text"])

Troubleshooting

Garbled or empty text — the format field probably doesn't match the actual bytes, or the audio is silent/corrupted. Confirm with ffprobe audio.wav.

400 with "Invalid base64" or silent failure — data must be just base64, not a data URI (data:audio/wav;base64,...). Strip the prefix if you copied it from a browser FileReader.

400 with a ZodError — a required field is missing or the wrong type. The body looks like {"success":false,"error":{"name":"ZodError","message":"[...]"}} — the nested message JSON string names the bad path (commonly input_audio.data or input_audio.format).

413 / request too large — base64 inflates bytes by ~33%, so a large raw file becomes an even larger JSON payload. Use a smaller source file (compressed format, lower sample rate, or trimmed clip).

Model not found — use the full slug from /api/v1/models?output_modalities=transcription (google/chirp-3, not chirp-3).

References

• STT guide
• Models page — filter to transcription output