Practitioner (own blog/paper) Language port / new library
Simon Willison ports an HTML5 parser from Python to JavaScript with Codex CLI in about 4.5 hours
Simon Willison had Codex CLI (GPT-5.2) port Emil Stenström's JustHTML Python library to dependency-free JavaScript. After a spec prompt and a small first milestone, he told it to 'do the rest, commit and push often' and left it running while he bought a Christmas tree. Eight prompts later it had produced 9,000 lines of tested JavaScript across 43 commits, passing the html5lib-tests suite.
The numbers (as reported)
| Idea to finished library | ~4–4.5 hours |
|---|---|
| Prompts | 8 |
| Output | 9,000 lines of JavaScript, 43 commits |
| html5lib tests passing | 9,200 (tokenizer 6,810/6,810; tree 1,770/1,782) |
| Estimated API-price cost | $29.41 (covered by $20/month ChatGPT Plus) |
| Tokens | 1.46M input + 97.1M cached input, 625,563 output |
Facts
- Company / subject
- Simon Willison (independent developer)
- Tools
- OpenAI Codex CLI, GPT-5.2
- Codebase
- greenfield
- Work type
- Language port / new library
- Stack
- JavaScript, Python
- Published
- 2025-12-15
- Last checked
- 2026-10-18
Caveats
Straight port of an existing library against a comprehensive existing test suite: close to ideal conditions. Ran with --yolo (no sandbox or approvals). Hit the Plus plan's five-hour token window and paused ~40 minutes. The author raises open questions on copyright, ethics and whether code built this way should be trusted in production.