Check a compose timeline before rendering

Finding out that a timeline was wrong used to mean submitting it, paying for the download and the probe, and reading the failure.

const check = await client.compose.validate(params);
if (!check.valid) console.error(check.problems);

It runs exactly the parse a render runs, costs nothing, and stores nothing. On success it returns the params as the engine reads them, with defaults filled in, so you can see what your timeline actually asked for.

• problems names what is wrong and where, by path.
• upgraded lists old fields it rewrote into the current shape.
• warnings lists settings the render will ignore: a video codec on an mp3, a size preset beside an explicit resolution, a key it does not know. None of them stop a submit.

Agents get the same check as an MCP tool, validate_compose, next to submit_job.

The endpoint and its response are in the compose docs.

It is the fastest way to check a timeline that uses the new effects list.