GPT Image 2.5 resolution: the size rules and the 4K ceiling

GPT Image 2.5 accepts custom output sizes inside one box: both edges a multiple of 16, the longer edge at most 3840 pixels, an aspect ratio no wider than 3:1, and a total between 655,360 and 8,294,400 pixels. The top of that box is 3840x2160, which is 4K. There is no fixed list of resolutions to pick from, and no resolution: "4K" tier on these models.

Sume's Image API docs state the same rules for openai/gpt-image-2.5 and openai/gpt-image-2.5-sunburst; OpenAI's image generation guide gives the same limits for the model. Both read 2026-09-29.

What sizes can I actually send?

Set image_size to a named preset, auto, or custom pixels as WIDTHxHEIGHT or { width, height }. Common targets, checked against the rules above:

Is 4K reliable?

It is supported, with a caveat from the vendor: OpenAI's guide says resolutions above 2560x1440 are experimental. On Sume, slow configurations such as 4K, high quality and several images per call are the ones most likely to return a 202 job envelope instead of an image, so read the status code, not the body shape.

Can I ask for a ratio instead of pixels?

Yes. aspect_ratio lists auto, 1:1, 16:9, 9:16, 4:3, 3:4, 5:4, 9:8 and 4:5 for these ids. Ratios that need custom pixels are converted to legal ones: 4:5 becomes 1024x1280, because 1080x1350 breaks the multiple-of-16 rule. When you edit a reference image, prefer aspect_ratio: "auto" so the output follows the source; leaving the field out is not the same as sending auto.

What if I need exactly 1920x1080?

These ids cannot produce it directly, since 1080 is not a multiple of 16. Generate at 1920x1088 and trim the extra 8 pixel rows in your own pipeline. Do not send resolution or size: "1080p" for pixel control: Sume's docs say custom pixels belong on image_size, not size.

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 studio shot of a ceramic mug on a wooden desk",
    "image_size": "1920x1088",
    "quality": "medium"
  }'