Alternative ending for a Short: write your own, not a preset

To write your own alternative ending for a Short, cut up to three clips of 3 seconds or less from it and send them to Gemini Omni Flash 1.1 as reference videos with a prompt of your own, 3 to 10 seconds long. YouTube's Extend with AI, as reported by Social Media Today, lets a creator pick a segment up to 5 seconds long and choose from suggested prompts, but not write a custom one.

The YouTube facts come from Social Media Today's report, read 2026-10-03, which is third-party. Sume's facts come from the video trim and Video Router docs.

What does Extend with AI let a creator do?

The report says the feature was announced in late September 2025 and works through the existing Cut option. A creator selects a segment up to 5 seconds long, picks from AI-generated prompt suggestions based on the content, and gets a new segment of typically 8 seconds or less that becomes a separate Short linked to the original. Music can be added from a curated list of tracks. The report lists the rollout as gradual to all countries outside the EU and UK, and states that custom endings are not possible.

• Segment in: up to 5 seconds.
• Segment out: typically 8 seconds or less.
• Prompt: chosen from suggestions, no custom text.
• Attribution: credited to the creator who generates it, with a link to the original.

How do I write my own ending on Sume?

Gemini Omni Flash 1.1 accepts reference_video_urls with up to 3 videos, each at most 3 seconds, plus up to 10 reference_image_urls. In the prompt you address them as <VIDEO_REF_0>, <VIDEO_REF_1> and so on, counting from zero in list order. A reference-video request carries no video_url, because that field is the edit mode and cannot be combined with references.

Cut the references with video trim: start plus exactly one of end or duration, so a 3-second moment is start: 12, duration: 3. Trim costs $0.02 per job and the output is a new MP4 that is never the source. Use precision: exact (the default) when the cut must land on a frame.

curl -X POST https://api.sume.com/v1/video-trim \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: ending-ref-0" \
  -d '{
    "video_url": "https://media.sume.com/artifacts/artf_demo/short.mp4",
    "start": 12,
    "duration": 3
  }'

What should the prompt say?

Describe what happens after the last reference moment, and name each reference so the model knows which subject you mean. For example: the character from <VIDEO_REF_0> opens the box on the table and finds it empty; hold the same room and lighting. Set duration between 3 and 10, aspect_ratio 9:16, and resolution 720p, the model's default, until you like the idea.

Synced audio is always on for this model and generate_audio: false is refused, so plan on a new audio moment in the ending. If you want your original soundtrack to carry through, see splicing an AI ending onto a Short.

How do the two routes compare?

The routes differ in who writes the prompt and where the file ends up.

What Sume does not do

Sume does not publish to YouTube from this flow, does not add the link-back, and does not promise that the generated ending keeps a character consistent. Review the clip, and keep YouTube's own disclosure and remix rules in mind if the source footage is not yours.