Seedance 2.5 duration -1: auto length vs Sume integer duration

On BytePlus, Seedance 2.5 takes duration as 4 to 30 or -1, where -1 lets the model choose the length. Sume documents duration as an integer number of seconds and seedance-2.5 as 4 to 30 seconds, so set a whole-second value yourself instead of sending -1.

Vendor behavior is from the BytePlus tutorial, read 2026-10-01. Sume behavior is from Video generation and Video Router.

What does -1 do on BytePlus?

The default is -1. For ordinary generation the model picks an integer length within 4 to 30 seconds from the prompt and references. For video editing the model keeps the output about as long as the selected video, possibly a non-integer length and up to 0.4 seconds shorter, and the page says editing supports only -1. The edited input must be 4 to 30 seconds long. For extension, ratio defaults to and only supports adaptive.

What does Sume ask for instead?

The request table lists duration as an integer, with the model range taken from the catalog. Aspect ratio is a value such as 16:9 or 9:16, and each model advertises the subset it accepts in supported_aspect_ratios. I found no -1 or adaptive value in those pages.

How do I port a duration -1 call?

Decide the length from your edit plan, send it as an integer from 4 to 30, and pick an explicit ratio from the model's list. If you would rather not choose a model at all, sume/auto lets Sume pick one. See which of Wan 3.0 and Seedance fits for the same question on another model.

What about clips that must match a source length?

Measure the source clip first and round up to a whole second inside the model's range. Because Sume documents integer seconds, a source of 6.3 seconds becomes a 7-second request; trim the result afterward if you need an exact match.