Timeline output size: 256 to 2160 per side, so no 3840x2160

Sume Timeline 1.0 accepts output.width and output.height as even integers from 256 to 2160, each side on its own. The default output is 1080x1920, a vertical MP4. That range is stated in the Timeline 1.0 docs, read 2026-10-05, and the video trim output conform uses the same 256 to 2160 band.

The consequence is that a 3840x2160 UHD render is outside the range, since 3840 is above 2160. A 4K-class vertical frame such as 2160x3840 is outside it as well, as 3840 is on the long side. The largest 16:9 frame that fits is 2160 wide by 1215, which is not an even height, so 2160x1214 is the nearest.

Sizes that do fit

What to do for a wider master

• Take the widest size that fits: 2160 wide, with the height rounded to an even number.
• Or keep the render at 1080 tall and set video[].fit to cover so the ultrawide fills the frame, accepting a crop.
• Set output.fps only when you need to; otherwise the job follows the sources.

Check without paying

POST /v1/timeline-1.0/plan is unbilled. It validates the schema and the URLs and returns the duration, segment count and estimated cost, so an invalid size surfaces before a render. A render costs $0.10 per started output minute, so a 61 second render bills two minutes.