Seedance 2.5 extend mode without ratio: how to continue a clip on Sume

Sume documents no mode: "extend" for Seedance. One way to continue a clip is to pass the last frame of the first clip as a first_frame entry in frame_images, and set aspect_ratio yourself. Runway's extend mode instead infers the ratio from the input video.

Runway facts are from its Dev changelog; Sume facts from the video generation docs, read 2026-10-01.

What is Runway's extend rule?

The August 7, 2026 entry says Seedance 2.5 video-to-video accepts mode: "reference" (default) or mode: "extend". Extend requires promptText and matches the input aspect ratio, so ratio may not be provided.

What does Sume offer instead?

Each frame_images entry carries a frame_type of first_frame or last_frame, and the model advertises the aspect ratios it accepts in supported_aspect_ratios. A video edit mode goes through the Video Router video_url field, which the docs describe for Gemini Omni Flash 1.1.

How do I keep the ratio consistent?

Read the aspect ratio of the clip you are continuing and send the same aspect_ratio on the next request. The docs do not say Sume infers the ratio from a frame image, so a mismatch is your client's to avoid.

How long can a continued sequence be?

seedance-2.5 accepts 4–30 seconds per request. Longer work is a chain of requests; the steps are in multi-round extension past 30 seconds.