Seedance 2.5 'extend twice': what Sume's 30-second request does

ByteDance's Seedance 2.5 page says you can 'create videos up to 30 seconds in a single generation, with the option to extend twice'. On Sume, seedance-2.5 takes 4-30 seconds in one request, but the catalog has no extend field and no way to hand a finished clip back to be extended. To go past 30 seconds you generate a second clip and join them.

The vendor wording is from Seedance 2.5 on ByteDance Seed, read on 2026-10-03. The page does not say how long each extension is, so this post does not either. Sume's range comes from the Video generation docs and the Video Router docs.

What maps and what does not

The single-generation half maps directly. Everything beyond it does not, and the table shows the difference in one place.

Sume's seed and unknown-field handling is strict: a field the model does not list, such as an extend or extensions key, is rejected with a 400 rather than ignored, so you find out at request time instead of after a bill.

Joining a second clip

The practical route to a longer sequence is a chain. Generate clip one, extract its last frame, and send that frame as the first_frame of clip two with a prompt that continues the action. The video-frames tool returns stills from a finished clip, and the timeline tool joins the clips into one file.

Expect a small visual seam. The model does not see the previous clip's motion or audio, only a still, so keep the camera move and lighting words identical in both prompts.

Cost of a chain

Each clip is billed on its own: list price times 1.25, by the model's own unit. Two 30 second generations cost what two 30 second generations cost; there is no discount for continuing. If the story is 45 seconds, generating 30 plus 15 is cheaper than 30 plus 30 and trimming, because you pay for seconds generated.

For a quick decision: if a story fits inside 30 seconds, ask for it in one request and skip the seam entirely. If it does not, plan the cut points before you generate, not after.