image_size or aspect_ratio on Sume: which wins and who accepts it

If you send both, image_size wins and aspect_ratio is ignored on models that take custom pixels. Sume's Image API says this field has priority over aspect_ratio. Send one of them per request, so a reader of your code can tell what the output size is meant to be.

size is a third field and a different thing. It is tier shorthand only (for example 2K) and rejects WxH with 400 unsupported_parameter. Custom pixels go on image_size.

Which field does what

GPT Image 2.5 pixel rules

For ChatGPT Image 2.5, custom pixels follow the OpenAI limits that Sume mirrors: both edges multiples of 16, longest edge at most 3840, aspect ratio at most 3:1, and 655,360 to 8,294,400 total pixels. OpenAI's own guide lists the same custom-size rule and notes that sizes above 2560x1440 are experimental (OpenAI guide, read 2026-10-07).

That is why Instagram portrait is not 1080x1350 on GPT: 1080 is not a multiple of 16. Ask for 4:5, or send 1024x1280.

Nano Banana is different

Nano Banana has no free-form pixels. A 1080x1350 value on image_size becomes aspect_ratio: "4:5" plus a job target_pixels hint. The native output is close to 928x1152 at 1K, and the exact 1080x1350 is a documented post-step, so plan a resize.

A request that is clear to read

Pick the ratio for models with a fixed list, and the pixels for models with custom sizes. Here is a Shorts-style cover on GPT, with pixels only:

{
  "model": "openai/gpt-image-2.5",
  "prompt": "bold title card, one subject, high contrast",
  "image_size": { "width": 1088, "height": 1920 },
  "quality": "medium"
}

Failure cases to expect

• size takes a tier shorthand and rejects a WxH value. Move the pixels to image_size.
• GPT custom pixels that break the multiple-of-16 rule, the 3:1 limit or the pixel window are rejected before submission, so keep a small validator next to your sizes.
• If both image_size and aspect_ratio arrive, image_size wins, so a stale aspect_ratio left in a template does not error and does not apply.

Check before you pin

The model's catalog row is the authority. GET /v1/images/models shows the aspect_ratio enum and whether the row accepts image_size. A parameter a model does not list is rejected, not dropped. See the Image API page for the full request table.