Remix cut refused with only a start time: the 120 s budget

A remix media cut that sends start without end is judged as if the clip ran to 300 seconds, so it fails remix_cut_too_long unless start is at least 180. The excerpt budget is 120 seconds in total, and the admission check fills a missing end with the 300-second source maximum instead of the real clip length.

This follows from the request schema on origin/main and the Remix media docs, both read 2026-10-11. The verb is meant for short evidence excerpts of a reference, not for producing the finished clip.

What cut accepts

cut takes an understanding_id from a completed probe and one of two selectors: start/end, or keep[]. It returns an MP4 of [start, end), or the kept spans joined on a fresh clock, plus a mapping[] that ties each source span to its place in the output.

• Total kept length is at most 120 seconds, summed across spans.
• keep[] holds 1 to 20 spans, each with start and end.
• label_time: true burns the source time into every frame, so a reader of the excerpt can say where in the original each moment sits.
• start/end and keep[] together return remix_cut_selector_conflict; an end at or before start returns remix_cut_range_invalid.

How the 120 seconds add up

The check runs at admission, using only the request. It cannot know the clip length from the body, so a missing start counts as 0 and a missing end counts as 300.

The fix is almost always an explicit end

Read the clip length from the probe, then send both ends. If you want the last 20 seconds of a 95-second reference, send start: 75 and end: 95. If you want several beats, put each in keep[] with its own span and keep the sum under 120 seconds.

A start-only request passes only when the implied span, 300 minus start, is 120 or less. That means start of 180 or more, which only makes sense for a long reference. Because the schema cannot see the clip, the safest habit is to never omit end.

When cut is the wrong verb

cut exists to hand an agent or a reviewer a labeled excerpt. For a production trim the docs point elsewhere: Video trim takes start plus one of end or duration, accepts sources up to 1,800 seconds and outputs up to 900 seconds, and lists a public rate of $0.02 per job.

A worked example with three beats

Suppose a 90-second reference ad has a hook at 0 to 8 seconds, a demo at 30 to 50 and a call to action at 80 to 90. As a hypothetical request, keep would hold those three spans and add up to 8 + 20 + 10 = 38 seconds, well inside the budget. With label_time: true, each frame shows where it came from in the original, and mapping[] lists the three source spans against their nominal output times.

The same idea scales to the full budget: twelve 10-second beats make 120 seconds and use 12 of the 20 spans. Eight more beats fit by span count, but not by seconds, so plan on whichever limit you hit first.

Availability

Remix media is flag-gated: the docs say development has it on and production is opt-in, so confirm the routes appear in your environment before you depend on them. Video trim is documented as available on dest and prod.