Nova Canvas 4,194,304 pixel cap and 16 px rule vs Sume image_size

Both Nova Canvas and GPT Image 2.5 on Sume refuse sizes that look normal, such as 1080 by 1350. They refuse them for different reasons, so a size you validated for one will not carry over.

Where the surprises are

On Nova Canvas, an input side does not need to be a multiple of 16 but an output side does. On Sume, 1080 by 1350 fails on GPT Image 2.5 for the same multiple-of-16 reason. See GPT Image 2.5 1080x1350 for the fix.

Check a size before you send it

This function tests a width and height against the Sume GPT Image 2.5 rules from the table. It is local, so a bad size never costs a request.

def gpt_size_ok(w: int, h: int) -> list[str]:
    errs = []
    if w % 16 or h % 16:
        errs.append("sides must be multiples of 16")
    if max(w, h) > 3840:
        errs.append("max edge is 3840")
    if max(w, h) / min(w, h) > 3:
        errs.append("aspect ratio must be at most 3:1")
    if not 655_360 <= w * h <= 8_294_400:
        errs.append("pixels must be 655,360 to 8,294,400")
    return errs

for size in [(1080, 1350), (1088, 1360), (1536, 864)]:
    print(size, gpt_size_ok(*size) or "ok")