Seedance 2.5 21:9 is 1470x630 at 720p: size, price and request

To make a 21:9 AI video on Sume today, send aspect_ratio: "21:9" to seedance-2.5 and you get 1470 x 630 pixels at 720p, 992 x 432 at 480p, and 2205 x 945 at 1080p. A 10 second 720p clip is billed at $5.81, against $5.78 for the same clip in 16:9.

Kling 4.0 lists 21:9 as a new ratio, according to the fal explainer (read 2026-10-05), and the full model is due in October. Sume's Kling model, kling-3, reports only 16:9, 9:16 and 1:1, so for wide frames the Seedance 2.5 and MiniMax H3 entries are the ones to use right now.

What the model returns

Seedance 2.5 accepts 4 to 30 seconds at 480p, 720p and 1080p, and its ratio list is 21:9, 16:9, 4:3, 1:1, 3:4 and 9:16, plus auto and adaptive. The Sume pricing estimator turns each resolution and ratio into a pixel size, then bills by video tokens. Wide frames are smaller per row than you might guess: the pricing tables give 720p 21:9 as 1470 x 630, not 1680 x 720, which keeps the pixel count close to the 16:9 tier.

Resolution still decides most of the bill. At 21:9 a 15 second clip is $4.03 at 480p, $8.71 at 720p and $21.43 at 1080p, so a draft at 480p costs less than half of the 720p pass. A sensible loop is to settle the prompt and camera move at 480p, check the framing of the wide crop, then spend the extra on one 720p or 1080p final. The tables below are estimates from the pricing package, so read the final amount from the job.

That is why price barely moves with ratio. 21:9 at 720p holds 926,100 pixels against 921,600 for 16:9, so the same clip costs a few cents more, not a third less.

Seedance 2.5 21:9 sizes and prices

Price and the one field to avoid

The figures come from the estimator in packages/provider-pricing and are what Sume reserves at submit. The final amount is usage.cost on the job. If you attach a reference video, the estimate changes; the reference video break-even post shows how.

Do not send size. The video docs state that each v1 model reports supported_sizes: null, so size returns 400 unsupported_parameter. Choose resolution and aspect_ratio instead, and read the allowed values from GET /v1/videos/models first.

A 21:9 request

This builds the request and prints it. It sends the job only when SUME_API_KEY is set, with an Idempotency-Key so a retry returns the original job.

import json, os, uuid

payload = {
    "model": "seedance-2.5",
    "prompt": "Slow dolly along a rain-soaked neon street at night",
    "aspect_ratio": "21:9",
    "resolution": "720p",
    "duration": 10,
}
print(json.dumps(payload, indent=2))

key = os.environ.get("SUME_API_KEY")
if key:
    import requests
    r = requests.post(
        "https://api.sume.com/v1/videos",
        headers={
            "Authorization": f"Bearer {key}",
            "Idempotency-Key": str(uuid.uuid4()),
        },
        json=payload,
        timeout=60,
    )
    print(r.status_code, r.json().get("polling_url"))

Cheaper wide options

If sound and cost matter more than the Seedance look, the MiniMax H3 entries also take 21:9. minimax-h3 runs 5 to 15 seconds at native 480p and 768p with native audio always on, and minimax-h3-max adds a 1080p tier that is a latent refinement of the 768p render. Both are far cheaper per second than Seedance 2.5 in the Sume estimator, so they suit a long wide storyboard where you iterate often.

Use Seedance 2.5 when you need more than 15 seconds in one shot, because the MiniMax models stop at 15. It also takes image, video and audio references, which the plain H3 entries cap at nine images, three videos and three audios, twelve references in total.

After you submit

The request is asynchronous. You get a job id and a polling URL, poll until completed, then download from the content URL, as the video generation docs describe. The jobs docs cover the same job on /v1/jobs/{id}/status.

Wan 3.0 and Kling 3 do not list 21:9. For a 16:9 clip that must become 21:9 afterwards, crop it rather than regenerating. When you join wide clips, set the Timeline output size yourself because the default canvas is 1080 x 1920 portrait.