GPT Image 2.5 background: opaque for listings, transparent

On Sume, the background field accepts auto, transparent or opaque, and only the two ChatGPT Image 2.5 models take it. For a marketplace listing photo, send opaque and ask for a plain white backdrop in the prompt. For a sticker or a layered design, send transparent with output_format: "png".

Which models accept the field

Sume's docs say background and mask_url are live only on ChatGPT Image 2.5, which has two API variants: Flare (the default) and Sunburst, per the launch report. Any other model that receives the field returns 400 unsupported_parameter, because the catalog gates parameters per model.

Two requests

The transparent call is the one to open in an editor and check on a checkerboard, since a faint halo around edges is the usual defect. Cutting out a real photograph is a different job; for that, see the background-removal route in the Sume API reference.

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":"ceramic mug, pure white seamless backdrop, soft shadow under the mug","background":"opaque","aspect_ratio":"1:1","quality":"high"}'

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":"die-cut sticker of a smiling cactus, thick white outline","background":"transparent","output_format":"png","quality":"medium"}'

What "white" means

The Sume docs do not promise exact pixel values for opaque, and the prompt, not the field, names the colour, so do not assume pure white (255,255,255). If a marketplace checks the corner values, sample them in your pipeline and reject any image that misses. Each marketplace sets its own rules, so read the one you sell on.

Limits

Sume's docs list background only for the two ChatGPT Image 2.5 models, so check the catalog before sending it to any other model. Quality defaults to high on GPT Image 2.5 if you omit it, which is the more expensive default, so pass quality deliberately on batches.

Defect checklist

Check these on each output:

• Opaque: corner pixels are the backdrop colour you asked for.
• Transparent: no halo on a dark and a light preview.
• Both: the file format matches the request.