On September 7, Simon Willison had a familiar problem: a demo video shot on his phone, too big to publish. The usual move is an online converter. Instead he asked Claude Fable 5.1, in Claude Code for web, to build him his own tool.
What came out of it
A video compressor that runs entirely in the browser. FFmpeg compiled to WebAssembly, so no file leaves the machine. It produces five variants in one pass, from Largest to Smallest, and lays them out sorted by file size.
The presets differ in resolution (854×370 or 640×276), CRF value (22 to 28) and audio bitrate (128 down to 64 kbps). On top of that: encoder speed, H.264 profile, fps limiting, metadata stripping, and dropping the audio track entirely. For every version it produces, the tool shows the FFmpeg command that made it.
It lives at tools.simonwillison.net/video-compressor.
Why this is more than a link
The compressor isn’t the interesting part. The decision before it is. Willison had a choice between “upload this file to somebody else’s service” and “have the tool built for me,” and the second one apparently wasn’t the more expensive option.
That shifts something. Small tools you need once or twice have always been a calculation: effort against payoff. When the effort collapses into a prompt, building it yourself wins nearly every time — and you get things web converters rarely offer. Your data stays local. The FFmpeg line sits right there, ready to copy into a terminal next time.
Exactly the kind of evening exercise I’ll be stealing. Not because of the video. Because of the question underneath it: which services am I still using that I no longer actually need?
Sources: Simon Willison: Video compressor, tools.simonwillison.net/video-compressor