Audio Extract API
Extracts the audio track from a video (or any container) and returns it in a chosen audio format.
- Endpoint
POST /v1/audio-extract- Price
- $0.003 per call
- Family
- Audio & video
01When to use it
- Demuxing and re-encoding an audio track needs ffmpeg AND a hardened fetch: an agent that pulls the URL itself walks past every SSRF control, and carrying an ffmpeg per call is not something an agent does inline.
When not to use it
- The input is over 200 MB (209,715,200 bytes) or longer than 60 min (3,600 s): audio-extract answers too_large (413).
- The destination needs more than 94 s to answer: the call ends with upstream_timeout (424).
- The destination is on a private or local network: the network guard refuses it with blocked_target (403).
02Parameters
| Field | Type | Required | Description |
|---|---|---|---|
| url | string | yes | Absolute http(s) URL of a video or audio file, up to 2048 characters, in a recognised container (MP4/MOV, Matroska/WebM, AVI, Ogg, WAV, FLAC, AIFF or MP3); the selected audio stream must last at most 3600 seconds. |
| stream | integer | no | Zero-based index among the audio streams only (0 is the first audio track), 0 to 63; an index the file does not have is refused as unprocessable. Default: 0. |
| to | string | no | Output format for the extracted track: wav (16-bit PCM), mp3 (LAME), flac, aac (ADTS stream) or opus (in an Ogg container); defaults to mp3. One of: "wav", "mp3", "flac", "aac", "opus". Default: "mp3". |
03Limits
| Limit | Value |
|---|---|
| Size cap | 200 MB (209,715,200 bytes) |
| Longest input | 60 min (3,600 s) |
| Timeout | 94 s |
04Example
The published example of audio-extract, verbatim: the request body and the response it returns.
Request
POST /v1/audio-extract
content-type: application/json
payment-signature: <base64 x402 payload>
{
"url": "https://grist.tools/samples/av/probe-clip.mov",
"stream": 0,
"to": "wav"
}Response
200 OK
payment-response: <base64 settlement receipt>
{
"source_url": "https://grist.tools/samples/av/probe-clip.mov",
"bytes": 33850,
"container": "mp4",
"stream_index": 0,
"format": "wav",
"audio_bytes": 32044,
"audio_base64": "UklGRiR9AABXQVZFZm10IBAAAAABAAEAQB8AAIA+AAACABAAZGF0YQB9AAAA4ADo..."
}Sample file: av/probe-clip.mov. Two-second QuickTime clip: 64x36 PNG video at 2 fps with a light square moving across a slate field, plus the three rising 500 Hz beeps of waveform-beeps.wav as mono 16-bit PCM at 8 kHz.
In the sample itself, the ftyp box names major brand qt, minor version 512 and compatible brands qt; the top-level boxes run ftyp, wide, mdat and moov, so the moov index comes after the media data; mvhd counts 2,000 ticks at 1,000 per second, 2 s; the 2 tracks are url with sample entry png and url with sample entry sowt. In the returned file: the RIFF header counts 32,036 bytes after itself, and the fmt chunk, in its own field order, declares audio format 1 (integer PCM), 1 channel, 8,000 Hz, 16,000 bytes per second, a block align of 2 bytes and 16 bits per sample. The data chunk right after it holds 32,000 bytes of samples, 2 s at that rate.
| Response field | Type | In the example |
|---|---|---|
| source_url | string | "https://grist.tools/samples/av/probe-clip.mov" |
| bytes | number | 33850 |
| container | string | "mp4" |
| stream_index | number | 0 |
| format | string | "wav" |
| audio_bytes | number | 32044 |
| audio_base64 | string | a string of 67 characters |
05Call it from code
JavaScript
// audio-extract: $0.003 per call, paid in USD Coin on eip155:8453 via x402
// Install: npm install @x402/fetch@2 @x402/evm@2 viem@2
// Save as client.mjs (ES module, Node 18+), export EVM_PRIVATE_KEY with the paying wallet's key in your shell, then run: node client.mjs
// The wrapper reads the 402 (payment-required), signs and retries with payment-signature.
import { wrapFetchWithPayment, x402Client, decodePaymentResponseHeader } from '@x402/fetch'
import { ExactEvmScheme } from '@x402/evm/exact/client'
import { privateKeyToAccount } from 'viem/accounts'
const account = privateKeyToAccount(process.env.EVM_PRIVATE_KEY)
const client = new x402Client().register('eip155:8453', new ExactEvmScheme(account))
const fetchWithPayment = wrapFetchWithPayment(fetch, client)
const body = {
"url": "https://grist.tools/samples/av/probe-clip.mov",
"stream": 0,
"to": "wav"
}
const res = await fetchWithPayment('https://grist.tools/v1/audio-extract', {
method: 'POST',
headers: { 'content-type': 'application/json' },
body: JSON.stringify(body),
})
console.log(res.status, await res.json())
const settle = res.headers.get('payment-response')
if (settle) console.log(decodePaymentResponseHeader(settle).transaction)Python
# audio-extract: $0.003 per call, paid in USD Coin on eip155:8453 via x402
# Install: pip install "x402[requests,evm]>=2.24,<3"
# The session reads the 402 (payment-required), signs and retries with payment-signature.
import os
from eth_account import Account
from x402 import x402ClientSync
from x402.http import decode_payment_response_header
from x402.http.clients.requests import x402_requests
from x402.mechanisms.evm.exact import register_exact_evm_client
from x402.mechanisms.evm.signers import EthAccountSigner
account = Account.from_key(os.environ["EVM_PRIVATE_KEY"])
client = x402ClientSync()
register_exact_evm_client(client, EthAccountSigner(account), networks="eip155:8453")
session = x402_requests(client)
payload = {
"url": "https://grist.tools/samples/av/probe-clip.mov",
"stream": 0,
"to": "wav"
}
res = session.post("https://grist.tools/v1/audio-extract", json=payload)
print(res.status_code, res.json())
settle = res.headers.get("payment-response")
if settle:
print(decode_payment_response_header(settle).transaction)curl
# audio-extract: $0.003 per call, paid in USD Coin on eip155:8453 via x402
# 1. Unpaid call: HTTP 402, the requirements in the payment-required header (base64 JSON) and in the body.
curl -i -X POST 'https://grist.tools/v1/audio-extract' -H 'content-type: application/json' -d '{"url":"https://grist.tools/samples/av/probe-clip.mov","stream":0,"to":"wav"}'
# 2. Same call with the signed payment (an EIP-3009 authorization, EIP-712 signed: it cannot be
# typed by hand). x-payment is accepted as the v1 alternative. HTTP 200 carries payment-response.
curl -i -X POST 'https://grist.tools/v1/audio-extract' -H 'content-type: application/json' -H 'payment-signature: <base64 x402 payload>' -d '{"url":"https://grist.tools/samples/av/probe-clip.mov","stream":0,"to":"wav"}'06Errors
| Code | HTTP | For this service |
|---|---|---|
| invalid_input | 400 | The body does not match the schema; its fields are url, stream and to. |
| blocked_target | 403 | |
| unreachable_target | 424 | |
| upstream_timeout | 424 | |
| unsupported_content_type | 415 | |
| too_large | 413 | |
| unprocessable | 422 | |
| internal | 500 |
Input that is rejected before the tool runs (malformed JSON, schema mismatch, body over the size limit) is never charged.
When each code is raised, and what it means for payment: /docs/audio-extract.
07Questions
- How much does a audio-extract call cost?
- $0.003 per call, paid in USDC on Base via x402, with no account and no API key.
- What does audio-extract return?
- A JSON object with 7 top-level fields: source_url, bytes, container, stream_index, format, audio_bytes and audio_base64. The example on this page is a real call over av/probe-clip.mov.
- What happens when a audio-extract call fails?
- The tool answers with a typed JSON error from the errors table. Payment is settled only after the tool has produced its result; a call that fails inside the tool is never settled. A 503 upstream_unavailable can follow a settled call; the x402 payments guide covers it, 429, 402 and an uncertain 500.