21:9 on kling-3 returns 400: which Sume video ids accept ultrawide

If you send aspect_ratio: "21:9" to kling-3 on Sume you get HTTP 400 with unsupported_capability and the message 'kling-3 does not support aspect_ratio 21:9', followed by the list it does take: 16:9, 9:16 and 1:1. For ultrawide, pin seedance-2.5 (or seedance-2), minimax-h3 or minimax-h3-max, which list 21:9 in the catalog.

Kling's own comparison page lists '21:9 ultrawide output' for Kling 4.0 and not for Kling 3.0 (kling.ai, read 2026-10-03). Kling 4.0 is not an id on Sume today. The Sume side is in the Video Router docs and the Video generation docs.

Aspect ratios by id

Only the ids with a ratio list in the catalog are shown. Lists are what the catalog reports; always confirm against GET /v1/videos/models because it is the live source.

Note two special cases. auto and adaptive on the Seedance and Wan ids let the model follow the first-frame image instead of forcing a ratio. And gemini-omni-flash-1.1 takes only 16:9 or 9:16.

Getting ultrawide without it

If you are committed to Kling, the fallback is to generate 16:9 and crop. A 21:9 frame is 2.33 wide to 1 high; a 16:9 frame is 1.78 to 1, so a crop keeps roughly three quarters of the height. Keep the subject in the middle band when you write the prompt. The video-filter tool can crop an existing clip.

Cropping loses pixels: a 1920x1080 frame cropped to 21:9 is 1920x823, so you lose resolution you paid for. Native ultrawide from a model that supports it is the cleaner route.

Pick the model first, then the ratio

Ratio is the first filter, not the last. A request that ignores the table fails before it queues, so nothing is billed, but a pipeline that retries blindly wastes wall-clock time. Read the list once at start-up and validate locally.

For vertical and square social work, kling-3 covers 9:16 and 1:1 without a problem; the ultrawide gap only matters for cinematic or banner uses.