The project file is the interface: letting AI agents drive a video editor

Last week I open sourced FableCut , a Premiere-style video editor that runs in the browser and that AI agents can operate. It hit the front page of Hacker News ( thread ), and the questions there made me realize the interesting part isn't the editor. It's one design decision: the project file is the interface. The usual way, and why I flipped it Most AI video tools hide the edit behind an API. You call addClip() , applyFilter() , and the tool owns the state. If you want a human to touch the result, you build a whole collaboration layer. FableCut does the opposite. The entire timeline lives in one JSON document, project.json : media, clips, tracks, keyframes, transitions, markers. The editor UI reads it. The export renders it. And anything that can write JSON can edit video: Claude Code through MCP, a Python script, jq , or you with a text editor. { "id" : "c_title" , "kind" : "text" , "track" : "V3" , "start" : 0 , "duration" : 2.2 , "props" : { "text" : "HANDMADE" , "font" : "Bebas Neue" , "glow" : 45 , "textAnim" : "letter-pop" } } That clip is a glowing kinetic caption. There is no API call that creates it. Writing it into the file IS creating it. SSE as a doorbell, not a data channel The first question on HN was "what's the benefit of SSE here?" Fair question, because the SSE channel does almost nothing, and that's the point. The server watches the project file with fs.watch , debounces 150ms, and pushes the literal string change to the browser. No payload. The browser re-fetches the project and re-renders. The whole mechanism is about 15 lines on a bare node:http server. Why not WebSockets? Because the data only flows one way. Everything that writes (the UI, an agent, a shell script) goes through REST or the filesystem. The browser only ever needs to hear "something changed, go look." An event with no payload can't arrive out of order, and a missed event costs nothing because the next fetch has the latest state anyway. The revision counter, or: how a human and