Music Router rejects duration: ask for a 28-second bed in the prompt

Sume's Music Router rejects duration and duration_seconds, so put the length in the prompt: write a 28-second track, or mark sections as [0:00-0:10] Intro. Each accepted generation is a fixed $0.125, so a wrong length costs a new generation, not a refund. Check the length of what comes back, and let the timeline soundtrack fade or loop the difference.

The Music Router docs (read 2026-10-06) say the prompt takes 1 to 5000 characters, that exclusions belong in the positive prompt because a non-empty negative_prompt is not supported, and that sume/music-auto currently picks Lyria 3.5.

What the request accepts

The surprise for people coming from other music APIs is the missing length field. Sume treats length as part of the creative brief.

A prompt for a 28-second holiday ad

Name the length, the energy and the section times, and keep it instrumental if a voiceover will sit on top: for example, a warm 28-second instrumental holiday bed with a soft intro from 0:00 to 0:06, a lift at 0:06, and an ending at 0:28, no vocals. The docs show section marks such as [0:00-0:30] Intro as the way to control structure.

Then measure the result. A prompt is a request, not a guarantee, so inspect the file before it goes in a render. If the track is shorter than the ad, the Timeline 1.0 soundtrack setting can loop it and fade out over up to 10 seconds; if it is longer, the timeline output length decides where the ad ends.

Tradeoffs

Generating again to fix a length is cheap at $0.125, but it adds a step and a different take. If one 30-second bed works across all your holiday cuts, generate it once and reuse it. If each cut needs its own length, ask for the longest you need and let the timeline cut the rest, which costs nothing extra beyond the $0.10 per started minute you already pay for the render.