How much does a subtitle-convert call cost?

$0.003 per call, paid in USDC on Base via x402, with no account and no API key.

What does subtitle-convert return?

A JSON object with 5 top-level fields: source_url, source_format, target_format, cue_count and subtitle. The example on this page is a real call over av/subtitle-captions.srt.

What happens when a subtitle-convert 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.

catalogue / tools / Audio & video / Subtitle Convert API

Subtitle Convert API

Converts a subtitle track between SubRip (.srt) and WebVTT (.vtt), normalising the cues.

Endpoint: POST /v1/subtitle-convert
Price: $0.003 per call
Family: Audio & video
Docs: /docs/subtitle-convert

01When to use it

It fetches through a hardened guard and re-serialises the cue model rather than string-patching, so positioning settings, cue ids and NOTE blocks convert cleanly instead of corrupting the output an agent would have to repair.
Your source is SRT or WebVTT and you need WebVTT or SRT.

When not to use it

The input is over 5 MB (5,242,880 bytes): subtitle-convert answers too_large (413).
The destination needs more than 15 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 SubRip or WebVTT file, at most 2048 characters; the source format is detected from the content, not the extension.
to	string	no	Target format, vtt (default) or srt; cues are re-serialised, so SRT output is renumbered and VTT cue ids, settings and NOTE blocks are dropped. One of: "srt", "vtt". Default: "vtt".

03Limits

Limit	Value
Largest source file	5 MB (5,242,880 bytes)
Timeout	15 s
Source formats	SRT, WebVTT
Output formats	WebVTT, SRT

04Example

The published example of subtitle-convert, verbatim: the request body and the response it returns.

Request

POST /v1/subtitle-convert
content-type: application/json
payment-signature: <base64 x402 payload>

{
  "url": "https://grist.tools/samples/av/subtitle-captions.srt",
  "to": "vtt"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/subtitle-captions.srt",
  "source_format": "srt",
  "target_format": "vtt",
  "cue_count": 3,
  "subtitle": "WEBVTT\n\n00:00:01.000 --> 00:00:03.500\nThe tide goes out slowly.\n\n00:00:04.000 --> 00:00:06.250\nShells line the wet sand,\none row after another.\n\n00:00:07.000 --> 00:00:09.000\nBy noon the water returns.\n"
}

Sample file: av/subtitle-captions.srt. SubRip subtitle file with three cues of neutral sample text, one of them spanning two lines.

The returned text starts with the line WEBVTT. Its 3 timing lines are 00:00:01.000 --> 00:00:03.500, 00:00:04.000 --> 00:00:06.250 and 00:00:07.000 --> 00:00:09.000, giving hours, minutes and seconds, and a full stop before the milliseconds. No cue has an identifier line above its timing.

Response field	Type	In the example
source_url	string	"https://grist.tools/samples/av/subtitle-captions.srt"
source_format	string	"srt"
target_format	string	"vtt"
cue_count	number	3
subtitle	string	"WEBVTT 00:00:01.000 --> 00:00:03.500 The tide goes out slowly. 00:00:04.000 --> 00:00:06.250 Shells line the wet sand, one row after another. 00:00:07.000 --> 00:00:09.000 By noon the water returns. "

05Call it from code

JavaScript

// subtitle-convert: $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/subtitle-captions.srt",
  "to": "vtt"
}

const res = await fetchWithPayment('https://grist.tools/v1/subtitle-convert', {
  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

# subtitle-convert: $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/subtitle-captions.srt",
    "to": "vtt"
}

res = session.post("https://grist.tools/v1/subtitle-convert", json=payload)
print(res.status_code, res.json())
settle = res.headers.get("payment-response")
if settle:
    print(decode_payment_response_header(settle).transaction)

curl

# subtitle-convert: $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/subtitle-convert' -H 'content-type: application/json' -d '{"url":"https://grist.tools/samples/av/subtitle-captions.srt","to":"vtt"}'

# 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/subtitle-convert' -H 'content-type: application/json' -H 'payment-signature: <base64 x402 payload>' -d '{"url":"https://grist.tools/samples/av/subtitle-captions.srt","to":"vtt"}'

06Errors

Code	HTTP	For this service
invalid_input	400	The body does not match the schema; its fields are url and to.
blocked_target	403
unreachable_target	424
upstream_timeout	424
unsupported_content_type	415	The source is not a type subtitle-convert reads; it reads SRT and WebVTT.
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/subtitle-convert.

07Conversions

Conversion	Recorded sample	Source size	Output size
SRT to WebVTT	av/subtitle-captions.srt	200 bytes	202 bytes
WebVTT to SRT	av/pair-captions.vtt	120 bytes	116 bytes

08Questions

How much does a subtitle-convert call cost?: $0.003 per call, paid in USDC on Base via x402, with no account and no API key.
What does subtitle-convert return?: A JSON object with 5 top-level fields: source_url, source_format, target_format, cue_count and subtitle. The example on this page is a real call over av/subtitle-captions.srt.
What happens when a subtitle-convert 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.

09Related

Audio & video guide