YouTube dynamic thumbnails: generate three options by API

YouTube Studio's dynamic thumbnails have YouTube recommend the best of three options to different audience segments, so you need three finished thumbnail files. On Sume, send three image jobs, each with its own Idempotency-Key, in webhook mode so every submit returns 202 straight away and you collect the files when each job finishes.

Read 2026-09-30. YouTube's post, as read, says nothing about how candidates are chosen, so this page covers only the file production. For choosing what differs between the three, see YouTube dynamic thumbnails: three for different audiences.

Why three separate jobs?

Each thumbnail has its own prompt, so each is its own request. Reference images go in image_urls (1 to 10 public HTTPS URLs) on Image 1.0. Sending the same references with three prompts keeps the subject constant while the framing or words change.

What mode should I use?

The Jobs and results page says webhook mode returns 202 with the job envelope and polling URLs, and the callback is stored. The job id is in the first response in every mode. Keep polling as a backup, since a delivery can fail.

What goes in each request?

A prompt, the reference URLs, and aspect_ratio. YouTube thumbnails are 16:9, so set "16:9". On edits, the Image API says to prefer "auto" to match the reference shape; omitting the field is not the same as auto.

curl -X POST https://api.sume.com/v1/image-1.0/generate \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: thumb-option-a" \
  -d '{
    "prompt": "Same subject, bold close crop, empty space on the left",
    "image_urls": ["https://media.sume.com/artifacts/artf_demo/frame.png"],
    "aspect_ratio": "16:9",
    "mode": "webhook",
    "webhook_url": "https://example.com/hooks/sume"
  }'

What happens after the three finish?

Each completed job returns Sume-hosted images under result.artifacts[]. Download the three, check faces and any lettering against your original, then use them in YouTube Studio. Sume does not upload to YouTube or run the test.