What does it cost to convert MP3 to FLAC?

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

How large can the MP3 file be?

Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. 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.

catalogue / tools / Audio & video / MP3 to FLAC

Convert MP3 to FLAC

To convert MP3 to FLAC, send one POST to /v1/audio-convert with the MP3 source and "to": "flac" in the body. Left out, "to" falls back to its default "mp3", so a FLAC 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 100 MB and 60 min (3,600 s) long.

Endpoint: POST /v1/audio-convert
Price: $0.003 per call
Service: audio-convert
Docs: /docs/audio-convert

01Limits

Limit	Value
Largest source file	100 MB (104,857,600 bytes)
Longest input	60 min (3,600 s)
Timeout	94 s
Source formats	AIFF, AVI, FLAC, MKV, MP3, MP4, Ogg, WAV
Output formats	AAC, FLAC, MP3, Opus, WAV

02Example

A real MP3 to FLAC run of audio-convert.

Request

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

{
  "url": "https://grist.tools/samples/av/pair-tone.mp3",
  "to": "flac"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/pair-tone.mp3",
  "bytes": 12980,
  "source_container": "mp3",
  "format": "flac",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 43000,
  "audio_base64": "ZkxhQwAAACICQAJAAAAAAAbWAfQBcAAAAAAAAAAAAAAAAAAAAAAAAAAABAAADgYA..."
}

Source file: av/pair-tone.mp3, 12.7 KB (12,980 bytes). 3 s mono 8 kHz MP3 at 32 kbit/s, encoded by ffmpeg's libmp3lame with the bit-exact flags: three 0.5 s 500 Hz triangle beeps at rising level, each followed by 0.5 s of silence. The recorded run returned 42 KB (43,000 bytes) of FLAC, 3.3 times the source size, with the source read as the mp3 container. Decoded, the FLAC in audio_base64 opens with 66 4C 61 43 00 00 00 22 02 40 02 40 (ASCII fLaC). Recorded with ffmpeg 8.1.2.

In the sample itself: the file opens with an ID3v2.4 tag with 10 bytes of body, then an MPEG 2.5 Layer III frame header at byte 20: 32 kbps, 8,000 Hz, mono. That first frame is an Info header rather than audio, and it counts 44 audio frames and 12,960 bytes. In the returned FLAC: its 34-byte STREAMINFO block, in its own field order, declares blocks of 576 samples, a largest frame of 1,750 bytes, 8,000 Hz, 1 channel, 24 bits per sample and a total sample count of 0, which the format reserves for a length the encoder did not write back. Its MD5 signature of the decoded audio is all zeros, the value for a signature that was not computed. The next metadata block is VORBIS_COMMENT, 14 bytes long.

03Format notes

MP3 as a source

MP3 is lossy audio. Grist accepts it when the file opens with an ID3v2 tag or with a bare MPEG audio frame sync: the byte FF followed by a byte whose top three bits are set. That sync test also matches a raw ADTS AAC stream, and because nothing proves ffmpeg would then pick the right demuxer, AAC is kept out of the sources. Duration needs care with MP3. For a file without a Xing header, the probe estimates length by dividing file size by the bitrate of the first frames, which a crafted variable-bitrate file can falsify, so any probe warning sends the measurement to a scan of real packet timestamps. Once accepted, the first audio stream is decoded by ffmpeg; a lossless destination stores the decoded samples without recovering what MP3 encoding discarded.

FLAC as a target

FLAC output is produced by ffmpeg's flac codec and flac muxer, streamed to a pipe rather than a temporary file. This destination is lossless, so a bitrate_kbps value is rejected instead of silently ignored. The caller may pin a sample_rate from the closed set; otherwise ffmpeg picks, normally the rate of the input. Before encoding starts the duration is checked, then only the first audio stream is mapped, video is discarded, and metadata and chapters are removed. Bit-exact flags make the output reproducible with the pinned ffmpeg. From a PCM source the result is normally smaller, and at the source rate the samples are preserved. From a lossy source it grows instead, because the lossless encoder stores every decoded sample, artefacts included, and cannot recover what the earlier compression removed.

04Errors

Code	HTTP
`invalid_input`	`400`
`blocked_target`	`403`
`unreachable_target`	`424`
`upstream_timeout`	`424`
`unsupported_content_type`	`415`
`too_large`	`413`
`unprocessable`	`422`
`internal`	`500`

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

05Questions

What does it cost to convert MP3 to FLAC?: $0.003 per call, paid in USDC on Base via x402.
How large can the MP3 file be?: Up to 100 MB (104,857,600 bytes), and at most 60 min (3,600 s) of media. 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.