Timeline refuses more than 8 chained fades: fix a vertical montage

Sume Timeline 1.0 refuses a request with more than 8 adjacent fades and returns too_many_chained_transitions. The fix is a hard cut: leave transition off one slot in the chain, and the render goes through.

The rule is listed in the refusals table of the Timeline 1.0 docs (read 2026-10-06): "More than 8 adjacent fades. Insert a hard cut." It matters most in fast vertical montages for Shorts and Reels, where it is tempting to fade every cut.

Which transition rules can a montage hit?

Several refusals live next to each other, so it helps to see them in one place.

How do I fix a nine-fade montage?

Say you have 12 slots and every slot after the first carries "transition": {"type": "fade", "duration": 0.25}. That is 11 adjacent fades, so the request is refused. Remove the transition key from slot 7 (and, in a longer montage, from every eighth slot after that). Slots without a transition cut hard, and the chain count resets.

Keep the declared start values as they are. The docs say declared starts are authoritative and the compiler compensates for the overlap, so removing a transition does not mean re-timing the other slots.

Does a long montage still render?

Yes. With render.strategy left at auto, Sume chunks the render past 12 segments. Only single is refused above 12 slots (render_strategy_unsafe). A 3-minute Short, which is YouTube's limit, can use up to 200 video[] slots.

Run the body through POST /v1/timeline-1.0/plan first. It runs the same schema checks and the pure compiler without creating a job or reserving credits, so a chained-fade error shows up before you pay for anything.