background: transparent on the Sume image API: two GPT rows only

To get a transparent background from the Sume image API, send background: "transparent" with output_format: "png" on openai/gpt-image-2.5 or openai/gpt-image-2.5-sunburst. These are the two catalog rows that list the background field. Any other model rejects the request with 400 unsupported_parameter, because Sume does not silently drop a parameter a model does not list.

The request

The call is a normal POST /v1/images. background accepts auto, transparent or opaque. PNG and WebP can carry alpha, JPEG cannot, so pick png here. Quality low is enough for a cutout test, and the billed rate at 1024-class size is 1 cent.

import os, requests

r = requests.post(
    "https://api.sume.com/v1/images",
    headers={"Authorization": f"Bearer {os.environ['SUME_API_KEY']}"},
    json={
        "model": "openai/gpt-image-2.5",
        "prompt": "a single red enamel pin, flat vector style, centered",
        "background": "transparent",
        "output_format": "png",
        "quality": "low",
    },
    timeout=60,
)
print(r.status_code)  # 200 with data[0].url, or 202 with a job envelope
print(r.json())

What it costs and what it does not cover

The price is the same whether the background is transparent or opaque: nothing in the catalog lists a separate transparency charge.

If your model has no background field

Nano Banana, Seedream, Flux, Ideogram and the other rows do not list background in this catalog. For those, generate the image and remove the backdrop in a second step, or route the transparent jobs to GPT Image 2.5. The older Image 1.0 URLs also accept transparency: true, but Image 1.0 is a compatibility alias for the Auto pipe and is retiring.

Always check the alpha channel of the saved file before you ship. A 200 response only tells you the image was generated, not that the corners are clear. Slow settings can return 202 instead, in which case you read the file from the job result.

Ways to check the result

Open the PNG in an editor with a checkerboard view, or read the corner pixel's alpha value with an imaging library. A fully opaque corner means the background was not removed. If you generate sticker or logo sets, sample the four corners and the center, and fail the job if any corner has alpha 255.

Remember that a transparent subject can still have a soft shadow baked into the pixels. Ask for a flat subject with no shadow and no ground plane in the prompt text, since the background field controls the canvas rather than the drawing.

At volume the arithmetic is simple. A sticker or icon set of 500 transparent PNGs at low costs $5, at medium $10 and at high $35 at 1024-class size. Run the first 10 at each tier, inspect the edges at 100 percent zoom, and move up only if the low tier shows fringing.