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.net
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.
Reach people exploring AI agents
See placements →What 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
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.