Nano Banana Pro custom aspect ratio: what Sume does

You cannot set a free-form ratio on Nano Banana Pro through Sume. The model has 11 native values (auto, 21:9, 16:9, 3:2, 4:3, 5:4, 1:1, 4:5, 3:4, 2:3, 9:16), and in today's code a ratio or a WIDTHxHEIGHT size that is not one of them is snapped to the nearest native ratio instead of being rejected. Exact pixel sizes are not served.

This is from the Image API docs and the API code, read 2026-09-29. Treat the snap as current behavior, not a contract.

Which aspect ratios does Nano Banana Pro take?

The catalog lists auto, 21:9, 16:9, 3:2, 4:3, 5:4, 1:1, 4:5, 3:4, 2:3, 9:16, where auto leaves the choice to the model. Nano Banana 2 has the same list plus 4:1, 1:4, 8:1, 1:8, which are extreme banner and skyscraper shapes that Pro does not list.

What happens to a custom ratio or size?

For the Nano Banana models the API reduces the request to a ratio and maps it to the closest native value. A WIDTHxHEIGHT in image_size is treated the same way, and the pixel size you asked for is recorded on the job as target_pixels but not applied.

How do I get an exact 1080x1350 image?

You do not get it from the model. The docs say Nano Banana Pro sends aspect_ratio: "4:5" for that size, at a native size of about 928×1152 at 1K, and that exact 1080×1350 is a post-step. Generate at 4:5, then resize or crop in your own pipeline.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "google/nano-banana-pro",
    "prompt": "A skincare bottle on a pale stone shelf",
    "aspect_ratio": "4:5",
    "resolution": "2K"
  }'

How do I check which ratio I actually got?

Download the image and divide its width by its height, then compare with the ratio you sent. If you sent a value that is not native, the shape you get is the snapped one, as in the table above. Sume keeps 4:5 and 3:4 distinct instead of mapping one to the other.

Copy the file into your own storage while you check it. Sume's image URLs are signed, so do not store the link as your record of the image.

Should I rely on the snap?

No. Send a native value so the request means what it says. Read supported_parameters from GET /v1/images/models and pick the closest ratio yourself, then crop. If your shape is a banner wider than 21:9, Pro cannot make it natively; Nano Banana 2 lists the extreme ratios above.

curl "https://api.sume.com/v1/images/models" \
  -H "Authorization: Bearer $SUME_API_KEY"