Lyria 3.5 has no duration field: how to ask for a 45-second track

You cannot set a duration on Sume's music router: duration and duration_seconds are rejected, so you ask for the length in the prompt, such as "a 45-second track". Every generation costs a flat $0.125, so a prompt that misses the length costs the same as one that hits it.

Prompt styles and what they cost

Two ways to express length, both inside prompt (1 to 5,000 characters):

A request that sets length and keeps the negatives positive

negative_prompt must be omitted or an empty string; a non-empty value fails with 400 negative_prompt_unsupported. Say what to leave out inside the prompt itself.

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: explainer-bed-01" \
  -d '{"prompt": "Calm corporate bed, 100 BPM. [0:00-0:15] Intro: soft piano. [0:15-0:45] Build: add strings, light pulse. A 45-second track. Instrumental, no vocals."}'

What to check

If you need an exact length, trim or loop in a follow-up step (Timeline audio is $0.01 per job) rather than regenerating.

• Measure the returned file; the length is a request, not a guarantee.
• Read result.lyrics when present for the section map the model reports.
• Read job.request.routed_model to see the engine that ran.