Music API duration_seconds rejected: set track length in the prompt

If you send duration or duration_seconds to Sume Music 1.0 or the Music Router, the API rejects the request. Neither field exists. The docs tell you to control length in the prompt, with phrases such as "a 2-minute track" or timed section markers like [0:00-0:30] Intro: .... Music 1.0 has no seed, temperature or guidance parameter either, so the prompt is the whole control surface.

Fields that exist and fields that do not

A length that fits a video

A fixed length matters when you must match a render. A 15-second reel at 128 BPM is eight bars, as a related post shows, so say both the tempo and the length: "A 15-second track, 128 BPM." The result is a prompt-level request, not a guarantee, so inspect the artifact. If the track is longer than the video, trim or let the timeline cut it. A track that is too short can be extended by timeline audio concat, which joins up to 20 parts without re-synthesis.

Remember the price is the same either way. $0.125 buys one accepted generation whatever the length you ask for in the prompt, according to the Music 1.0 page, so there is no saving from asking for a short track.

Where the 400 shows up

The error appears at request time, before a job exists. Check your SDK or wrapper first, because generic media clients often add a duration field by default. Remove it, move the number into the prompt, and keep the Idempotency-Key stable so the retry maps to the same job.

• Prompt maximum: 5000 characters.
• Music Router unknown model ids fail with 400 model_not_found and a catalog_url.
• Read the result from result.artifacts[] where type is audio (usually audio/mpeg).

Checking length after the fact

Because length is a request in words, check what you got. Download the audio artifact from media.sume.com and read its duration with your usual tool. If you need the exact length for a timeline, trim in the render step instead of regenerating. Timeline audio can split a file into ranges at $0.01 per job, which is cheaper than a second $0.125 generation.