Convert MP4 to WebVTT (subtitle track)

To convert MP4 to WebVTT (subtitle track), send one POST to /v1/subtitle-extract with the MP4 source and "to": "vtt" in the body. Left out, "to" falls back to its default "srt", so a WebVTT request has to send it. Each call costs $0.003 in USDC on Base via x402, with no account and no API key. The source file can be up to 200 MB.

Endpoint

POST /v1/subtitle-extract

Price

$0.003 per call

Service

subtitle-extract

Docs

/docs/subtitle-extract

01Limits

02Example

A real MP4 to WebVTT run of subtitle-extract.

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

{
  "url": "https://grist.tools/samples/av/pair-clip.mp4",
  "stream": 0,
  "to": "vtt"
}

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-clip.mp4",
  "bytes": 11315,
  "container": "mp4",
  "stream_index": 0,
  "format": "vtt",
  "cue_count": 2,
  "subtitle": "WEBVTT\n\n00:00.000 --> 00:01.750\nRain taps on the window.\n\n00:02.250 --> 00:03.750\nA lamp glows in the hall.\n"
}

Source file: av/pair-clip.mp4, 11 KB (11,315 bytes). 4 s MP4 made by ffmpeg: a 64x36 PNG video stream at 1 fps (a slate field with a small amber square moving right), a mono 8 kHz FLAC audio track of two 500 Hz triangle beeps, and an English mov_text subtitle stream with two cues. The request sets stream to 0. The recorded run returned 108 bytes of WebVTT, 1% of the source size, with the source read as the mp4 container, stream index 0 and 2 cues. Recorded with ffmpeg 8.1.2.

In the sample itself, the ftyp box names major brand isom, minor version 512 and compatible brands isom, iso2 and mp41; the top-level boxes run ftyp, free, mdat and moov, so the moov index comes after the media data; mvhd counts 4,000 ticks at 1,000 per second, 4 s; the 3 tracks are vide with sample entry mp4v, soun with sample entry fLaC and sbtl with sample entry tx3g. The returned text starts with the line WEBVTT. Its 2 timing lines are 00:00.000 --> 00:01.750 and 00:02.250 --> 00:03.750, giving minutes and seconds with no hour field, and a full stop before the milliseconds. No cue has an identifier line above its timing.

03Format notes

MP4 as a source

MP4 belongs to the ISO base-media family, and Grist recognises it by the ftyp box type at bytes four to eight rather than by a file extension. QuickTime MOV and other files carrying the same box are sniffed as this one container, and the response names it mp4. The container can hold video, audio and text subtitle streams side by side, and each service takes one kind. audio-convert maps the first audio stream and drops the picture. video-thumbnail puts the seek before the input, so it happens at the demuxer and stays cheap in a large file, then decodes exactly one frame from the first video stream. subtitle-extract selects a text track by its zero-based position among subtitle streams. Each ffmpeg run strips container metadata and sets bit-exact flags, so outputs do not carry creation timestamps.

WebVTT as a target

subtitle-convert writes WebVTT as the WEBVTT header, a blank line, then cues with dotted timestamps in HH:MM:SS.mmm form. WebVTT is that service's default destination. Output is rebuilt from each cue's start, end and text, so SubRip index numbers disappear and no positioning settings are added. Text subtitle tracks inside media containers take a different writer: subtitle-extract passes them through ffmpeg's webvtt muxer, and in the recorded extraction results that muxer prints timestamps without an hour component. Metadata is stripped and bit-exact flags are set on that route, so the same stream yields the same text for a pinned ffmpeg. Both routes end in the same format, while the serialiser, and therefore the exact timestamp layout, depends on whether the captions started as a file or as a track. Nothing follows the end timestamp on a timing line, because subtitle-convert writes no cue settings, and no identifier precedes a cue.

Note: The container must carry a text subtitle track; image-based subtitles cannot be turned into text.

04Errors

When each is raised, and what it means for payment: /docs/subtitle-extract.

05Questions

What does it cost to convert MP4 to WebVTT?

$0.003 per call, paid in USDC on Base via x402.

How large can the MP4 file be?

Up to 200 MB (209,715,200 bytes). Past a limit the call answers too_large (413).

What happens if the conversion 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.

06Related

• Audio & video guide
• AVI to WebVTT
• MKV to WebVTT
• Ogg to WebVTT
• SRT to WebVTT