GPT Image 2.5 banner: 3840x1280 passes the 3:1 rule

3840x1280 is a valid custom size on ChatGPT Image 2.5 in Sume: it is exactly 3:1, both edges are multiples of 16, the long edge equals the 3840 maximum, and 4,915,200 pixels sits inside the allowed range. It is a usable wide-banner size for a hero strip or a header.

The four rules, checked

The Image API docs give four limits for custom pixels on the 2.5 rows: both edges multiples of 16, maximum edge 3840, aspect ratio at most 3:1, and 655,360 to 8,294,400 total pixels.

Where it stops

A 4:1 strip does not pass. 3840x960 is 4:1, and 3840x1200 is 3.2:1; both break the ratio limit. If you need a longer strip, generate at 3:1 and extend it outside the model, or compose two images on a timeline. Sume does not offer a native 4:1 size on these rows.

• Use image_size with custom pixels; do not put pixels on size.
• Pin quality, because auto reserves the max price.
• Send aspect_ratio or image_size, not both, to avoid an ambiguous request.

A request

The body below is the shape the docs use, with the custom size added as the docs describe.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-image-2.5",
    "prompt": "Wide website hero banner, soft morning light, empty left third for text",
    "image_size": {"width": 3840, "height": 1280},
    "quality": "medium"
  }'