WebP with transparency: output and import rules

Synthesia's September 29, 2026 update says WebP files can be brought into its editor with transparency preserved. On Sume, choose the format with output_format and ask for a transparent background with background: "transparent" on a model that supports it. The docs do not say that a WebP result keeps its alpha channel, so open the file and check.

What Synthesia says

The Synthesia updates page lists, for Sep 29, 2026, that WebP files can be brought into the editor with transparency preserved. That is an import rule for their editor. It does not tell you what any generator exports.

Sume output formats

Image 1.0 documents output_format as png, jpeg, jpg or webp. The Image API page lists png, jpeg, webp or svg, and says background accepts auto, transparent or opaque on ChatGPT Image 2.5 (openai/gpt-image-2.5). For transparent stills the page also points to Image 1.0 with transparency: true. Image 1.0 is retiring and is a compatibility alias for the Auto pipe.

A request that asks for both

This asks for a transparent background in PNG. Swap in webp only after you have confirmed that your pipeline keeps the alpha channel.

curl -X POST https://api.sume.com/v1/images \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: transparent-sticker-001" \
  -d '{
    "model": "openai/gpt-image-2.5",
    "prompt": "A flat sticker of a red kettle, clean edges, no shadow",
    "background": "transparent",
    "output_format": "png"
  }'

Check, then import

Download the result from the Sume artifact URL and open it on a checkerboard background in a viewer that shows alpha. Check content_type on the artifact: it tells you which format Sume actually returned. Only then upload it to the destination tool.

Retries should reuse the same Idempotency-Key. Poll the job and read result.artifacts[] for the media.sume.com URL.