Editor Beat Sync

Requires

A running Scrollmark editor whose commands you can call: the SocialGPT editor chat, or scrollmark mcp from @scrollmark/cli for a coding agent. The beat ops read the music clip's beats inside the editor. Without it, only the rules (never move a speech cut, downbeats for reveals) carry over.

The two ops

Two timeline_applyPlan ops put edits on the music's beats; the plan command lists them by name only, and their arguments are here:

• beatCut {clips: [handles in play order], music: the music clip, every?: "auto" | "bar" | 1 | 2 | 4 | 8}
• snapToBeat {clips: [handles], music: the music clip, to?: "beat" | "downbeat"}

Both need a music clip on the timeline: use the one that is there, or add one in the same plan with a music op named with "as": "$bed" and pass "$bed" as music. A bed looped from a short track is fine: the beats of every copy are read.

Cut shots to the beat: beatCut

For a run of shots (B-roll, a montage, several clips with no speech):

{"op": "beatCut", "args": {"clips": ["c2", "c3", "c4", "c5"], "music": "c9"}}

• clips in the order they should play, all on one track. On the main track the first starts where it is; on an overlay track it starts on the nearest beat. Every change after it lands on a beat.
• every: leave it out ("auto") and each shot keeps about its length, in whole beats; that is what "cut to the beat" means. "bar" cuts only on downbeats, for calm or slow music. A number (1, 2, 4, 8 beats a shot) only when they ask for fast or punchy cuts; that shortens the montage, so end the music with it (timeline.setClipDuration on the music clip, then a fade-out). A shot too short for its beats takes the most whole beats it has.
• Never on a talking head or interview: moving a speech cut onto a beat clips words. Leave speech cuts where the transcript put them and put overlays on beats instead.

Put moments on beats: snapToBeat

After placing titles, B-roll overlays, stickers or SFX, move their starts onto the music's beats in the same plan:

{"op": "snapToBeat", "args": {"clips": ["$title"], "music": "c9", "to": "downbeat"}}

• to: "downbeat" (the first beat of a bar) for a hook, a title or a big reveal; "beat" for everything else.
• It moves starts only and keeps lengths; main-track clips are refused, so use it on overlays.

When there is no beat

If either op says the music has no steady beat, tell the user the track has no clear beat to cut to and offer a different track; do not place cuts by guesswork. Read beat times yourself (audio_detectBeats, in the audio group) only when an op cannot do what was asked.

Tell the user the tempo (the plan result's beatCut or snapToBeat entry) and how many beats each shot runs.