Convert FLAC to MP3
To convert FLAC to MP3, send one POST to /v1/audio-convert with the FLAC source and "to": "mp3" in the body. "mp3" is the schema default of "to", so the field may also be left out. 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
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 FLAC to MP3 run of audio-convert.
POST /v1/audio-convert
content-type: application/json
payment-signature: <base64 x402 payload>
{
"url": "https://grist.tools/samples/av/convert-beeps.flac",
"to": "mp3"
}200 OK
payment-response: <base64 settlement receipt>
{
"source_url": "https://grist.tools/samples/av/convert-beeps.flac",
"bytes": 32086,
"source_container": "flac",
"format": "mp3",
"bitrate_kbps": null,
"sample_rate": null,
"audio_bytes": 2180,
"audio_base64": "SUQzBAAAAAAACgAAAAAAAAAAAAD/4xjEAApAAx75QRACiHZUlw94AlAQBAEwfB8H..."
}Source file: av/convert-beeps.flac, 31.3 KB (32,086 bytes). Two-second mono 16-bit FLAC at 8 kHz holding the same three rising 500 Hz beeps as waveform-beeps.wav, written with uncompressed verbatim subframes by a TypeScript generator. The recorded run returned 2.1 KB (2,180 bytes) of MP3, 7% of the source size, with the source read as the flac container. Decoded, the MP3 in audio_base64 opens with 49 44 33 04 00 00 00 00 00 0A 00 00. Recorded with ffmpeg 8.1.2.
In the sample itself: its 34-byte STREAMINFO block, in its own field order, declares blocks of 4,096 samples, no largest frame size, 8,000 Hz, 1 channel, 16 bits per sample and 16,000 samples in total. In the returned MP3: it opens with an ID3v2.4 tag with 10 bytes of body, then an MPEG 2.5 Layer III frame header at byte 20: 8 kbps, 8,000 Hz, mono.
03Format notes
FLAC as a source
A native FLAC stream begins with the four ASCII characters fLaC, and that marker is what Grist sniffs. FLAC compresses audio losslessly, so decoding restores the exact samples that were encoded. audio-convert is the only service that reads the format directly, and its own canonical example decodes a FLAC file to a bit-exact PCM WAV. FLAC audio can also sit inside other containers as a stream, but such files are recognised by their container signature and reported under that container, not as flac. Before encoding, the stream length is measured and checked against the service's duration limit. Like every audio-convert source, only the first audio stream is used, metadata and chapter information are stripped, and the pinned ffmpeg with bit-exact flags makes the result reproducible. A lossy destination discards information the FLAC still held.
MP3 as a target
MP3 is the default value of audio-convert's to field, encoded with the pinned libmp3lame codec. The mp3 muxer needs no seeking, so output streams straight to a pipe instead of a temporary file. bitrate_kbps applies to this lossy destination; when it is omitted, no audio bitrate argument is passed, the encoder's own default applies, and the response reports the bitrate as null. An optional sample_rate selects from the closed set, otherwise ffmpeg normally follows the source. Only the first audio stream is encoded, and video, chapters and metadata are left out, with bit-exact flags keeping the bytes reproducible for the pinned ffmpeg. Even with source metadata removed, each recorded MP3 output still begins with an ID3v2 tag written by the muxer. Inputs whose duration exceeds the service limit are refused before encoding.
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 FLAC to MP3?
- $0.003 per call, paid in USDC on Base via x402.
- How large can the FLAC 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.