Build a 3-minute YouTube Short from clips with Sume Timeline

To build a three-minute Short from several clips, send one Timeline 1.0 render with audio.duration_seconds set to 180 or less, one video[] slot per clip, and a soundtrack bed if you have rights to the music. At the documented $0.10 per output minute, a 180-second render is three billed minutes, $0.30. YouTube's own rule is a Short up to three minutes, square or vertical, and the default Timeline output is 1080 by 1920.

The YouTube rule is from Understand three-minute YouTube Shorts and Upload YouTube Shorts, read 2026-10-02. Timeline facts are from the Timeline 1.0 docs.

What is the spine and why does it set the length?

Timeline is built on an audio spine. audio.duration_seconds (1 to 1800) is required and decides the output length; video slots are placed on that timeline. The first video[] slot must start at 0, later starts must increase, each slot is at least 0.2 seconds, and coverage may trail the spine by at most 0.5 seconds.

So the 180-second ceiling for a Short is a number you set, not a Timeline limit. Set it to the real length of your voice-over or music and the video follows it.

How do transitions and the music bed work?

Transitions go on slots after the first: fade, wipeleft, wiperight, slideup, slidedown or dissolve, at most one second and at most half the shorter neighbour. Output fades (fade_in_seconds, fade_out_seconds) run 0 to 5 seconds and their sum cannot exceed the length.

A soundtrack is an optional bed with url, gain_db, loop, fade_out_seconds up to 10 and duck_db from 0 to 20, which lowers the bed under the voice. Ducking needs a real spine, not silence. The bed must be music you are allowed to use; YouTube's three-minute page says most songs can run up to 90 seconds in a three-minute Short, though some tracks are limited to 60 or 30 seconds.

What does a render call look like?

This renders two clips and a voice track into 100 seconds. Add slots to reach your length, and keep each slot's start increasing. The default output is 1080 by 1920, so you do not need an output block for a vertical Short.

If the render uses more than 12 segments, render.strategy auto chunks it; forcing single above 12 slots is refused.

curl -X POST https://api.sume.com/v1/timeline-1.0/render \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: long-short-001" \
  -d '{"audio":{"url":"https://media.sume.com/artifacts/artf_demo/voice.wav","duration_seconds":100},
       "video":[{"source_url":"https://media.sume.com/artifacts/artf_demo/a.mp4","start":0,"duration":50},
                {"source_url":"https://media.sume.com/artifacts/artf_demo/b.mp4","start":50,"duration":50,
                 "transition":{"type":"fade","duration":0.5}}],
       "soundtrack":{"url":"https://media.sume.com/artifacts/artf_demo/bed.mp3","duck_db":12,"loop":true,"fade_out_seconds":3}}'

What should I check before upload?

Read warnings[] on the result: padded or looped short sources and a frame-rate resample are reported, not failed. Run video inspect with frames: false and confirm duration_seconds is 180 or less and the frame is vertical.

Sume does not publish the file, and it cannot say whether your music is cleared. That responsibility, and the Content ID outcome, stay with you and YouTube's current rules.