What does it cost to convert FLAC to Opus?

$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.

catalogue / tools / Audio & video / FLAC to Opus

Convert FLAC to Opus

To convert FLAC to Opus, send one POST to /v1/audio-convert with the FLAC source and "to": "opus" in the body. Left out, "to" falls back to its default "mp3", so a Opus 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 FLAC to Opus run of audio-convert.

Request

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

{
  "url": "https://grist.tools/samples/av/convert-beeps.flac",
  "to": "opus"
}

Response

200 OK
payment-response: <base64 settlement receipt>

{
  "source_url": "https://grist.tools/samples/av/convert-beeps.flac",
  "bytes": 32086,
  "source_container": "flac",
  "format": "opus",
  "bitrate_kbps": null,
  "sample_rate": null,
  "audio_bytes": 17130,
  "audio_base64": "T2dnUwACAAAAAAAAAAAAAAAAAAAAAGS5xGoBE09wdXNIZWFkAQE4AUAfAAAAAABP..."
}

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 16.7 KB (17,130 bytes) of Opus, 53% of the source size, with the source read as the flac container. Decoded, the Opus in audio_base64 opens with 4F 67 67 53 00 02 00 00 00 00 00 00 (ASCII OggS). 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 Opus: the first Ogg page, flagged as the beginning of the stream, has stream serial 0 and one 19-byte packet, the OpusHead: version 1, 1 channel, a pre-skip of 312 samples, an input sample rate of 8,000 Hz and channel mapping family 0.

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.

Opus as a target

An Opus result is libopus-encoded audio inside an Ogg container: the ogg muxer writes it, and the recorded outputs begin with the OggS capture pattern followed by an OpusHead identification header. For that reason a returned Opus file is recognised as ogg if it is submitted again, and opus never appears as a source slug. bitrate_kbps controls the lossy encode, and an optional sample_rate may be chosen from the closed set; left out, ffmpeg normally follows the source rate. The input's duration is checked first, then the first audio stream alone is encoded, excluding video, metadata and chapters. Pinned ffmpeg and bit-exact flags make repeated calls return the same bytes. With no bitrate given the encoder's default rate applies, and for the recorded low-rate mono examples that default produced larger files than the MP3 default.

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 Opus?: $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.