Porting JustHTML from Python to JavaScript with Codex CLI in an afternoon
Simon Willison gave Codex CLI a handful of high-level prompts to port JustHTML to a dependency-free JavaScript library. He published the repo and a playground.
The problem
Porting a library between languages by hand is tedious. An existing shared test suite makes it work an agent can complete with little supervision.
How they did it
- Point the agent at the original Python library and the html5lib-tests suite.
- Give a few high-level prompts and let the agent loop until the tests pass.
- Review the commits and publish the result as simonw/justjshtml.
Results
- The library passes 9,200 tests from the html5lib-tests suite.
- The agent produced 9,000 lines of fully tested JavaScript across 43 commits.
- The title reports 4.5 hours of agent run time.
As reported by the source (Simon Willison's Weblog); AgentGid did not measure these figures.
This suits a solo developer or small team that already has a mature shared test suite and can read a few thousand lines of generated code, since the review of 43 commits is the real work. Watch the 4.5 hours of run time: it is the case's own reported figure, and on API-key billing the token cost is not stated. Pi (Free, OSS) is a cheaper alternative, though you pay your model provider.
The agent used here
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