Claude Code builds a local video compression interface
Willison wanted to optimize a phone recording before publishing it. He used Claude Code for web to build a browser tool around FFmpeg compiled to WebAssembly, with multiple output presets.
simonwillison.netWhat happened
The source shows a tool producing several encoded versions with previews and downloads; no general speed claim is made here.
Reported by Simon WillisonHow the agent did it
- Use the need to compress a specific phone recording as the implementation goal.
- Build a browser interface around the WebAssembly version of FFmpeg.
- Generate different encodings and compare previews and file sizes to choose a usable output.
Where a person stepped in
Willison chose the recording and evaluated output options. Publication approval boundaries are not described.
A person approved each step before the agent continued.
More details
About this account
The coding agent built the tool from the delegated goal; the short account links the session but does not detail its test loop.
Firsthand tool note and screenshot document use for a real recording. Quality and timing shown are specific to that input.
Credit: Simon Willison
Added 14 Oct 2026
Source published 7 Sept 2026
Source checked 14 Oct 2026
Tools and topics
Claude Code · FFmpeg · WebAssembly
Browser · Media
Editorial completeness: 100/100. Measures documented fields, not agent performance or verified outcomes.
Prompt to try
Editorial adaptation, not the source author’s exact prompt. Replace bracketed inputs and review tool permissions first.
Build a local video compression page using [approved FFmpeg build]. Offer a few clearly labeled quality presets, output sizes and previews. Test with a non-sensitive sample, show the actual command for each output and report unsupported browsers. Keep processing local; do not upload the input.