Music Router duration_seconds: 4 models take it, 5 do not

Four of the nine Music Router models now accept duration_seconds: elevenlabs-music-2.5 (10–300), stable-audio-2.5 (5–190), ace-step (5–240) and cassetteai-music (10–180). The other five, sume/music-auto, lyria-3.5, lyria-3-pro, minimax-music-2.6 and lyria-2, still reject the field, and duration is refused on every row.

Older advice that the router has no length field is out of date for those four ids. This post is the current rule, read from Sume's docs and from the code that ships on main.

Which ids take the field

Sume's Music Router page says duration_seconds is the track's length in whole seconds, "only on models whose catalog row lists capabilities.duration_seconds". Each of the four defaults to 60 when you omit it.

Whole seconds only: a value outside the row's range, or a fraction, fails validation with a message that names the model and its range.

Request that pins a length

Pin an explicit id and send the field next to the prompt. This one asks Stable Audio 2.5 for a 45-second bed.

Send Idempotency-Key as on any Music Router call, then poll /v1/jobs/{id}/result and read the audio artifact.

curl -X POST https://api.sume.com/v1/music-router/generate \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: bed-45s-001" \
  -d '{
    "model": "stable-audio-2.5",
    "prompt": "Calm ambient pad bed, 70 BPM, no vocals",
    "duration_seconds": 45
  }'

What to do on the other five

On Music Auto and the Lyria rows, put the length in the prompt ("a 2-minute track" or [0:00-0:30] Intro: … section marks). Lyria 2 writes one 30-second clip. MiniMax Music 2.6 writes its own length.

If you need an exact cut from one of those, generate a little long and trim on a Timeline instead of expecting the field to work.

Pricing follows the length on three of the four

elevenlabs-music-2.5 and cassetteai-music bill per started minute and ace-step per second, so duration_seconds changes the credits; stable-audio-2.5 is one price per track whatever the length. Each row's current credits are in GET /v1/music-router/models and on the API rate card.

The catalog row also lists credits_per_minute on the length-priced rows, which is the number to multiply when you plan a batch of long tracks.