Claude Fable 5.1 Wrote an FFmpeg-in-WebAssembly Video Tool in One Prompt
Video compressor is a browser-based tool built by Claude Fable 5.1 that uses a WebAssembly build of FFmpeg to compress video entirely client-side.
What it is
A browser tool from Simon Willison that runs FFmpeg compiled to WebAssembly to shrink video files without uploading them to a server.
What it does
It takes a locally recorded video and re-encodes it to an optimized, smaller file entirely in the browser, using the same FFmpeg compression logic normally run as a native binary.
Why it matters
Willison built this by having Claude Fable 5.1 in Claude Code for web generate the tool directly from a prompt, an instance of a coding agent producing a working WASM-based utility with no separate backend, on par with hand-built tools he's shipped before.
How to use it
Open the tool in a browser, drop in a video file, and download the compressed output; no install or server needed.
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