FableCut × WebMCP

A browser video editor that you and your agent edit at the same time.

Entry to The WebMCP Challenge

FableCut is a Premiere-style non-linear video editor that runs entirely in the browser. This repository adds a WebMCP layer on top of it, so an AI agent running in the browser tab can drive the editor directly: read your selection, move your playhead, cut your timeline, and land every change on the same undo stack you use.

There is one timeline, one history, and two people editing it.

What is new here

FableCut already existed before this hackathon, and it already had an MCP server. That server edits project.json, a file on disk. An agent using it is editing a document, and has to be told what everything is called.

The WebMCP layer edits the running session. Three of its tools read state that has no on-disk representation at all:

And the property that makes two-way editing usable rather than alarming:

Every mutating tool calls pushUndo() first. The agent's edits go onto the same undo stack as yours. Ctrl+Z reverses the agent exactly the way it reverses you. There is no separate "review the AI's changes" panel, because there is nothing separate to review.

Full tool list and the prior-work / new-work split: WEBMCP.md.

Related MCP server: cutgent

Try it

Live demo: https://ronak-create.github.io/fablecut-webmcp/

Open it in Chrome 149+ with chrome://flags/#enable-webmcp-testing set to Enabled, or in the built-in browser of the ChatGPT desktop app. Then ask your agent things like:

• "What is on this timeline?"

• "Move to 4.5 seconds and split whatever is under the playhead."

• "What is visible at the 8 second mark?"

• (select a clip yourself) "Make this one a second shorter."

• "Actually, undo that."

Without WebMCP the editor still works normally, and the layer logs a warning instead of registering.

Demo video: https://youtu.be/zooqpnJIXbE — an agent reading the timeline, cutting to beat markers, and being undone with Ctrl+Z off the shared undo stack. Cut in FableCut itself.

Run it locally

Two ways, both with zero dependencies and no build step.

Full editor (import your own media, ffmpeg export, the stdio MCP server):

node server.js          # http://localhost:7777

Static demo (what gets deployed, no server at all):

node build-demo.js      # produces dist/
npx serve dist

The static build is not a cut-down copy of the app. It is the same app.js, with static-mode.js serving the API surface from static files and localStorage. A hosted demo cannot share one project.json across visitors, because two judges opening the URL would be dragging each other's clips around. Removing the server entirely means every visitor gets a private copy of the demo timeline, with no shared state, no cold start, and nothing to run.

Add ?reset to the URL to throw away your edits and reload the pristine demo.

Layout

Provenance

The first commit in this repository is an unmodified export of ronak-create/FableCut at commit 4f555d1, containing no WebMCP code. Every commit after it is work done for this challenge, so:

git log --reverse --oneline    # the first entry is the baseline
git diff <baseline> --stat     # everything built for the challenge

The demo timeline is built only from assets that ship with FableCut under MIT (the vector animations in library/svg/) plus text and adjustment layers, which need no media at all. No client work and no third-party audio are redistributed here.

Licence

MIT. See LICENSE.