Why sume/auto returns 400 for 21:9, not a Seedance route

A common surprise: you send model: "sume/auto" with aspect_ratio: "21:9" and get a 400, even though Seedance on Sume accepts 21:9. This is deliberate. Sume's contract says sume/auto has its own capability envelope and fails closed instead of changing models behind your back.

What auto accepts

Why it fails closed

The resolution of sume/auto is a pure function of the request and the catalog version. There is no load balancing and no A/B, so an idempotent replay gets the same route and the same price. If it quietly picked Seedance for 21:9, the price would change under you: a 10-second 720p Seedance 2.5 clip is $5.78 on Sume, and the price of an auto clip is not that number.

What to send instead

• For 21:9 or any length from 11 to 30 seconds, name the model: seedance-2.5 takes 4 to 30 s and 21:9.
• For up to 10 seconds in 16:9 or 9:16, keep sume/auto.
• The error names sume/auto, not the resolved model, and its supported list shows the values that are accepted.

{
  "model": "seedance-2.5",
  "prompt": "A wide shot of a coastal road at dusk",
  "duration": 12,
  "resolution": "720p",
  "aspect_ratio": "21:9"
}