FableCut MCP server

A browser video editor that AI agents can drive.

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FableCut is a Premiere-style non-linear video editor that runs entirely in your browser — and exposes its whole timeline as one JSON document. Edit it by hand, from the UI, or let an AI agent (Claude Code, Claude Desktop, or anything that speaks MCP/REST) cut your video for you while you watch the timeline update live.

Zero npm dependencies. One node server.js. That's it.

Sponsors

Sidrune AI — One API for GPT, Claude, and other leading AI models. Sign up and get $3 in API credit.

Why it's interesting

Most "AI video" tools hide the edit behind an API. FableCut flips that: the project file is the interface. project.json describes media, clips, tracks, effects, keyframes and transitions — any process that can write JSON can edit video, and the open browser UI hot-reloads within ~150 ms via server-sent events. A human and an agent can work on the same timeline at the same time.

Taking back what the agent did. Every agent edit is one undo step of its own: Ctrl+Z takes back the agent's last change first (and says so), then goes on to yours, so the two never get mixed. For bigger rollbacks there are Versions: one is kept when an agent starts editing (again at most every 10 minutes while it works), before it rewrites the whole project, before every big operation (cutting silences, snapping to the beat, reframing), and every 10 minutes while you edit. Restore one from the Versions panel, or ask the agent (fablecut_versions), and compare any version with now first.

Features

Editing

• Video + audio tracks (default 3+4; add more with +V / +A in the track header; right-click an empty header → Remove track), drag/trim/split/snap, undo/redo
• Settings (cog in the top bar) — optional prefs stored in this browser via localStorage. Enable Link timeline and Project bin selection so picking a timeline clip highlights its media in Project, and clicking a Project item selects every timeline clip that uses it (off by default).
• Video + audio tracks (default 3+4), drag/trim/split/snap, undo/redo
  • +V / +A in the track header add a video or audio lane (up to 16 each)
  • Right-click an empty track header → Remove track (disabled if the lane has clips, or if it would leave you with zero video/audio tracks)
  • Track header S solos that lane (mutes all others); click again to restore the previous mute state. Using the mute toggle while soloed exits solo.
  • Track targeting — click a track's name to target / untarget it (lit = targeted). Split, insert, ripple, close gap and jump-to-cut touch targeted tracks only; the eye / speaker toggle is output-only (hide from preview and export) and no longer blocks edits. A selected clip is edited wherever it sits.
  • Track lock — the padlock in the track header. Nothing on a locked track can be moved, trimmed, split or deleted, and ripples leave it in place while the other tracks shift — lock the music bed and re-cut the picture freely.
• Clip lock, disable and unlink — right-click a clip (or use the toggles at the top of the inspector): Lock pins it like a locked track; Disable (⇧E) keeps it on the timeline but hides it from preview and export; Unlink (Ctrl/Cmd+L) splits a video from its audio so they move separately, and Link re-joins them once they line up again. Agents see locks too: fablecut_patch_project refuses to edit locked clips unless the op says force: true.
  • Dropping a video with more audio channels than A-tracks adds the missing lanes automatically (up to 16) and toasts how many were added; each channel becomes a linked stem on its own A-track
• Direct manipulation on the monitor — click a clip or title on the preview to move, resize (corner handles), or rotate (top handle, Shift-snap) it directly
• Timeline multi-select — rubber-band marquee (drag on empty track area), Ctrl/Cmd/Shift+click to add/remove clips, Ctrl+A to select all, Esc to deselect. Drag any selected clip to move the whole group; Delete removes all selected; S splits all selected at the playhead. Inspector shows an "N clips selected" banner.
• Beat & cue markers (tap m on the beat during playback) — name and colour them, drag them on the ruler, jump with ⇧m / Alt+⇧m or the Markers list; snapping targets (clip edges, playhead, markers, IN/OUT, keyframes, frame grid) are picked from the ▾ beside Snap
• Press Alt+t to add an in/out transition based on the playhead position over the selected clip. The last used transition is remembered as the default. Drag the overlay triangle to adjust duration; Delete clears the focused transition.
• Real decoded audio waveforms on clips
• Mixer — the Mixer tab beside the Inspector has a strip per audio track (fader, pan, mute, solo, meter) and a master fader with a live LUFS readout. Preview and export run through the same mix, so what you hear is what renders.
• Clip gain, channels and Normalize — each audio clip has a Gain (dB, before volume and keyframes), a Channels mode (stereo, mono, left, right, swap) and Normalize: measure the selected clips (ITU-R BS.1770 loudness or sample peak) and set their gain to hit −14 / −16 / −23 LUFS or −1 dBFS. Linked stems share one gain so a stereo pair stays balanced.
• Volume line, fades and crossfades — every audio clip shows its volume as a line you drag; Ctrl/Cmd-click adds a keyframe, Alt-click removes one. Corner grips drag fades in and out, drawn with their real curve (constant power, constant gain or exponential). Shift+D crossfades the cut next to the selection, borrowing spare media from both sides so nothing else moves.
• Auto-duck — select the music, pick the dialogue track and an amount, and the music dips wherever someone speaks (ramping down just before, back up after). It rides on top of the music's own volume, so re-running or clearing it never touches your levels. Agents use fablecut_auto_duck.
• Audio effects and presets — EQ, high/low-pass, compressor, limiter, noise gate, delay, reverb, distortion, stereo width and pitch shift, chained per clip, per track (Mixer → FX), per bus and on the master. Presets: Clean voice, Podcast, Radio, Deep voice, Telephone, Cinematic, Wide and Muffled, all tweakable afterwards. Preview and export run the same effects. Agents use the setFx patch op.
• Effect automation — the ◆ beside an effect setting keys it at the playhead, like clip keyframes: sweep a filter into the drop, open up a reverb at the end. Agents use setFxKeys.
• Submix buses — + Bus in the Mixer adds a bus with its own effects, fader, pan, mute and meter; route tracks into it from the menu under each track's name (all the dialogue through one compressor and one fader).
• Noise reduction — the inspector's Noise control (Light / Medium / Strong) measures the file's noise floor and renders a cleaned copy with ffmpeg; the clip's audio switches to it and Off switches back. Agents use fablecut_denoise.
• Reframe — make a 9:16, 1:1 or 4:5 copy of your edit whose shots follow the subject: a virtual camera pans after what matters in each shot and writes ordinary position keys you can still edit. From New sequence… or the Inspector's Reframe section; agents use fablecut_reframe.
• Highlights — the best moments of long footage (movement, sound, a clear picture) cut into a short edit in a new sequence, in story order and never across a cut. From the sequence menu's Highlights…; agents use fablecut_highlights.
• Silence removal — the inspector's Silence section (or right-click → Find silences) listens to a take and marks every pause on the timeline; Cut extracts them all in one undo step, keeping a little quiet beside each phrase so words aren't clipped. Agents use fablecut_silence.
• Beats — the inspector's Beats section finds a music clip's tempo, a steady beat grid and the drop, and shows the beat across the timeline. Mark the beats, snap every cut onto the nearest one, or re-cut a track so each shot lasts 1, 2, 4, 8 or 16 beats. Agents use fablecut_beats.
• Transcripts and text-based editing — the Transcript tab turns speech into words with times, in the browser (Whisper; the model downloads once and the audio never leaves your machine). Click a word to go there, select words and press Delete to cut them, or find a phrase and cut every time it is said. Agents use fablecut_transcribe to read an interview, find a line and cut it.
• Captions — one click makes captions from the transcript, timed word by word: subtitle, karaoke, a highlighted word or big popping words, sized for the canvas. Export or import SRT and WebVTT. Agents use fablecut_captions.
• Project bin folders — tree view with expand/collapse; drag media or folders to nest; right-click the Project tab → New folder; drop files onto a folder to import into it
• Media management — each file's frame rate, codecs, audio channels and size (ffprobe); search by name, codec, resolution or #tag; filter by favorites, used / unused, kind, tag or collection; sort by any detail or by times used; a list view with metadata columns; favorites, tags and collections; Find timeline instances and Reveal in project; offline files flagged with a "Media offline" slate, Relink… (finds moved files by name and size) and Replace file…
• Projects — a dashboard of every project with pictures, notes and tags: open, create from a canvas or a saved project template, duplicate, rename, archive and delete, import an old project file; media is shared, so copies cost no footage
• Sequences — several timelines in one project (a 16:9 and a 9:16 cut, cut-downs, alternatives) sharing its media and compositions, each with its own canvas, frame rate, mix and delivery settings; switch from the menu at the left of the timeline toolbar
• Versions — non-destructive history: save a version (Ctrl/Cmd+S), automatic versions every 10 minutes and before an agent edits, compare a version with now, restore (the current state is kept as a version first), mark the final cut; backups carry every version, and an unreadable project.json is recovered at startup
• Proxies — light, frame-accurate H.264 copies of 4K / HEVC / ProRes footage made in the background (ffmpeg); edit on the proxy with the monitor's Proxy button, export always renders the originals
• Import from URL — + URL downloads an HTTPS video/audio/image into ./media/ (same-origin after import). Remote SVG is refused. Agents use fablecut_import_media with an https:// path. The URL is not kept as media.src — that would taint the canvas and break export.
• Audio Hold — timeline toolbar toggle: while paused, loops one frame of audio at the playhead (useful when stepping frame-by-frame). Scrubbing or frame-step retargets the held slice; meters stay live. Play / Pause turns it off.
• Canvas aspect presets (16:9, 9:16 reels, 4:5, 1:1) + project FPS select (24 / 25 / 30 / 50 / 60; non-preset rates show as Custom) + safe-area guides
• Export frame / reframing — composition canvas can be larger than the delivery crop (exportFrame in project.json). Preview dims the overscan; drag the Export frame handle to reframe (e.g. 16:9 canvas → 9:16 export). Fast export crops to the frame; WebCodecs and Realtime export are disabled while a frame is set
• Program Monitor zoom — mouse-wheel over the preview zooms the composition toward the cursor (fit → up to 2 screen pixels per canvas pixel). Magnified view uses native scrollbars so overflow stays reachable; middle-click or Alt+drag pans. The Fit button (shown while zoomed) resets to the fit-to-stage baseline
• Full J/K/L shuttle — L plays forward, J in reverse; tap again for 1.5×/2×/4×, the other key turns around, K stops. Hold K and tap J/L to step a frame, or hold both to crawl at ¼ speed. Preview only, never the export
• Typed timecode — click the playhead readout (or type digits) to jump: 01:15:00, 1500, +30, 12.5; the IN / OUT readouts take typed times too
• Resizable workspace: drag the divider between monitor and timeline (double-click resets), plus S/M/L timeline track-density presets (S hides thumbnails for compact tracks)
• Zoom to selection (⇧Z) frames all selected clips, not just one
• IN/OUT work area — set markers with i and o (⇧I / ⇧O to clear). The Program Monitor shows playhead as current / sequence duration; when markers are set, IN, marked duration, and OUT stack on the right. Export has a Range dropdown (Entire timeline / IN–OUT; defaults to IN–OUT when markers exist) so you can keep markers for split/trim and still export the full sequence. Enabling Limit constrains playback to the marked range and maps Home / End to the IN and OUT positions rather than the full timeline. t splits clips at the markers; ⇧t trims clips to the work (between marker in and marker out) area.
• Find & close gaps — a gap is a stretch where every targeted track is empty (black frames). g jumps the playhead to the next shared gap (wraps; respects IN/OUT when both are set). ⇧G closes the gap under the playhead by pulling later clips left on all targeted tracks (locked clips stay put).
• Trim tools — a tool picker in the timeline toolbar (or V B R Y U): Selection, Ripple edit (drag a clip edge and everything after it follows, so no gap opens), Rolling edit (drag a cut between two clips; nothing else moves), Slip (change which part of the source a clip shows without moving it) and Slide (move a clip while its neighbours trim to make room). Every tool respects source length, targeting, locks and linked audio, and shows the offset beside the pointer while you drag. Agents run the same edits — split, ripple delete, lift / extract, insert / overwrite, ripple / roll / slip / slide and crossfade — as fablecut_patch_project ops, on the same code.
• Lift / Extract — ; removes the IN→OUT range on the targeted tracks and leaves the gap; ' removes it and closes the gap. Both also sit in the timeline toolbar.
• Jump to cut — ↑ / ↓ move the playhead to the previous / next edit (clip In or Out). With a clip selected, the first taps land on that clip’s start then end (Premiere-style); with no selection they walk cuts on targeted tracks. In the Source monitor they jump among 0, In, Out, and duration. Left/right still step frames; Home/End still go to the sequence (or IN/OUT with Limit).
• Ripple delete — the timeline Ripple delete button (or ⇧Del) removes the selection and pulls later clips left on each targeted track to close the gap; plain Del still lifts (leaves a gap). Linked AV partners always move together, even on untargeted tracks (sync lock); locked clips are never deleted or moved.
• Reset a property — Ctrl/Cmd+click an inspector label or slider restores that effect/prop to its default and clears every keyframe on the channel (scale → 1, opacity → 1, paired fields like Crop L/R reset together; transition labels clear the in/out transition). Shift+click a label is playhead-local: if you are parked on a keyframe it removes that keyframe only; otherwise it sets the value at the playhead to the default (auto-keys if the channel is already animated).
• Replace media — the inspector's Source button (any video/audio/image/svg clip) swaps the underlying file while keeping position, trim, keyframes, transitions and every effect. Pick another item already in the bin or Browse file… to import and replace in one step. A video's linked L/R audio companions are swapped along with it; a shorter replacement clamps the trim to fit and toasts that it did so.
• Multi-channel video audio — a video with more than 2 audio channels gets a linked audio clip per channel, not just L/R (5.1, 7.1…). Extra audio tracks (A5, A6, …, capped at 16) are created automatically as needed; replacing a clip's media re-syncs the linked channel clips to the new source's channel count, adding/dropping extras and new tracks as needed.

Look

• 14 one-click filter presets (cinematic, teal-orange, noir, vintage, cyberpunk, sunset, midnight…)
• Adjustment layers — one clip grades everything below it, Premiere-style
• Color workspace (top bar → Color): live waveform, RGB parade, vectorscope and histogram beside the program monitor, and a Color tab that grades the selected clip on the GPU — lift / gamma / gain / offset wheels, exposure, white balance with a picker (click something white), blacks / shadows / midtones / highlights / whites, log-style contrast with a pivot, soft rolloff and saturation, plus curves — luma and R / G / B, hue vs hue, hue vs saturation, hue vs luma and saturation vs luma, with a picker that marks a colour from the monitor, and layers (secondaries): a correction limited by an HSL qualifier and / or an ellipse, rect, polygon, bezier or free-hand mask (feathered, invertible, keyframable, dragged on the monitor), with a matte view. Copy / paste a grade across shots. Agents grade with the setGrade patch op and read the result as numbers with fablecut_scopes (levels, clipping, colour cast)
• Quick filter sliders: brightness/contrast/saturation/hue, temperature & tint, blur, grayscale/sepia/invert, vignette, animated film grain
• 17 blend modes (screen, multiply, overlay, color-burn, exclusion, hue, luminosity…), fit modes (contain/cover/stretch), per-edge cropping, corner radius, flip H/V
• Masks — isolate any part of a clip (video, image, SVG, text or an adjustment layer): rectangle, ellipse, bezier pen and free-hand shapes, several per clip, combined with add / subtract / intersect / difference, each with feather, expansion, invert and opacity, all keyframable (outlines morph). Draw and edit them on the monitor, with a matte view. Agents use the setMask patch op and measure with fablecut_scopes
• Shapes and motion graphics — shape layers of rectangles, ellipses, polygons, stars, lines and pen-drawn bezier paths in groups, with fill, stroke, dashes and trim paths for draw-ons; null objects, parenting and an anchor point on every clip; every value keyframes, edited right on the monitor. Agents use the setShape and setParent patch ops
• Motion paths, motion blur and text on a path — bend a clip's travel into a curve with handles on the monitor, turn it along the way, blur fast moves with a shutter angle (preview and export), and set titles along lines, arcs, circles or drawn paths. Agents use the setMotionPath and setTextPath patch ops
• Compositions — nested sequences: build a lower third, a logo sting or a scene once and place it anywhere as one clip that moves, masks, grades and blends like a picture, while its video and audio play and export as usual. Open one to edit it on the timeline, or precompose a selection. Agents use the addComp, inComp and precompose patch ops
• Animation presets and templates — 33 ready-made entrances, exits, emphases and loops that fit any clip where it sits, and templates (lower thirds, logo animations, 9:16 / 1:1 / 16:9 social layouts) whose text, colours and logo you fill in, each placed as its own comp. Save your own animations and comps to the library for every project. Agents use the applyPreset, addTemplate and setTemplate patch ops
• Motion tracking — follow a point or a region (with its scale and rotation) through a video clip, then make a mask follow it or pin a title, sticker or any clip to it; the result is ordinary editable keys. Agents use fablecut_track
• Track mattes — cut a clip with the picture on the track above by its alpha or its luma (inverted or not): text filled with video, shape reveals, texture cut-outs. The matte track hides itself. Agents use setMatte
• Chroma key (green screen) with tolerance/softness + spill suppression
• AI background removal (person cut-out, in-browser via MediaPipe)

Motion

• Keyframe animation on ~25 properties with easing
• Keyframe markers on clips — diamonds on the clip body at each unique keyframe time (tooltip lists channels; a count badge when several share a time). Ctrl/Cmd+← / Ctrl/Cmd+→ jumps the playhead to the previous / next keyframe (selected clips first, else clips under the playhead). Inspector fields show the interpolated value at the playhead; changing one updates the keyframe you’re on, or inserts one if that channel is already keyed. Dragging a clip in the