Wan 3.0 and Seedance 2.5 auto duration on fal: send seconds on Sume

Send duration as a whole number of seconds on every Sume video request. fal's own pages let the model choose a length when you leave it out, and that is convenient for exploring but a bad default for a budget, because you do not know the price until the clip exists.

What the vendor pages say

fal's Wan 3 page, read 2026-10-05, says durations run from 2 to 30 seconds in a single generation and that you can leave the length unset to let the model choose one. fal's Seedance 2.5 page, read the same day, lists durations as "auto" or 4 to 30 seconds.

On Sume, the catalog field supported_durations is a list of whole seconds, and duration is an integer. The video generation docs describe the catalog as the source of truth: a value it does not advertise returns a 400 instead of being adjusted.

What a fixed length costs

Because Sume reserves the price when you submit, the useful number is the price of the length you plan to ask for. Wan 3.0 is billed per second and Seedance 2.5 per video token; the Seedance 720p row is 16:9.

Why this matters for retries

Sume supports an Idempotency-Key header, and a replay returns the original job. A fixed duration means the replayed body is identical, so the key stays valid and the price stays the same. An unset duration is a value you do not control, and two submissions of the same prompt can legitimately differ in length.

If a clip comes out longer than you need, you pay for the extra seconds. If you ask for 6 seconds and want 10, you regenerate. Picking the number first avoids both.

A simple rule

Pick the duration from the shot, not from the model: 4 to 6 seconds for a product beat, 8 to 10 for a scene, 15 or more only when the model supports it (Wan 3.0 and Seedance 2.5 go to 30 on Sume). Then read the catalog row for the id, confirm the number is in supported_durations, and submit.