Tall 1:3 strip on GPT Image 2.5: 1280x3840 passes, 1280x3856 fails

For a tall strip on GPT Image 2.5, the biggest legal 1:3 size with a 1280 edge is 1280x3840. It is 4,915,200 pixels and exactly 3:1, which is the documented limit. 1280x3856 is only 16 pixels taller, but its ratio is 3.01:1 and Sume rejects it. Use the same logic in reverse for a wide strip.

Rules that apply

The documented custom-size rules (as of 2026-10-08) are: Both edges must be multiples of 16, the longest edge is at most 3840, the aspect ratio is at most 3:1, and the total is 655,360 to 8,294,400 pixels. A tall strip is limited by the 3:1 rule first and by the 3840 edge second. At a 3840 height the widest strip you can ask for is 1280 pixels.

If you want less height, scale in multiples of 16 on both edges. 1024x3072 and 1152x3456 are both exact 1:3 sizes and legal.

Other rows for tall ratios

Not every row needs custom pixels. Nano Banana 2.1 lists 1:4 and 1:8 as aspect_ratio values, and Ideogram V3 and 4.5 list 1:3. If your layout needs a ratio taller than 1:3, those rows are the way to get it, because the GPT rows stop at 3:1.

Read the aspect_ratio descriptor of the row on GET /v1/images/models before you pin a ratio. A model accepts only the values its catalog lists.

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

Validate

Run this before submitting.

def legal(w, h):
    px = w * h
    return (w % 16 == 0 and h % 16 == 0 and max(w, h) <= 3840
            and max(w, h) / min(w, h) <= 3 and 655_360 <= px <= 8_294_400)

for w, h in [(1280, 3840), (1280, 3856), (1024, 3072)]:
    print(w, h, w * h, legal(w, h))

Tall and wide ratios side by side

The 3:1 limit is symmetric, so everything here holds for a wide strip with the edges swapped. The table lists the largest legal size for tall ratios, plus the smallest at 1:3.

A note on 1080-wide layouts

Designs often start from a 1080 px width, but 1080 is not a multiple of 16 (67.5 x 16). Round to 1088 and crop two 4 px edges in your own tooling, or ask for 1024 and scale up. 1088x3264 is a legal exact 1:3 size at 3,551,232 pixels, so a 1080-wide scroll panel fits with a small trim.

Validate the size before you send. Both edges must be multiples of 16, the longest edge is at most 3840, the ratio is at most 3:1, and the total is 655,360 to 8,294,400 pixels. A check of those four rules in your own code turns a 400 into a message you control, and it costs nothing. Run it on user-entered sizes in particular, since a person typing a height will often overshoot by a few pixels, as 3856 does here. If the number fails, snap the long edge down to the nearest multiple of 16 that keeps the ratio at 3:1 or flatter, and show the user the result.