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Codex ports a Python HTML parser to JavaScript

Willison supplied an existing parser and an independent compatibility suite, reviewed an API plan, then let Codex implement a JavaScript port through repeated tests and commits.

Added simonwillison.netAgent: Codex
TRY A BOUNDED VERSION

Prompt to try

Editorial adaptation, not the source author’s exact prompt. Replace bracketed inputs and review tool permissions first.

Port [library] to [language] in a separate directory. Inspect its license, API and independent compatibility suite. Write a plan and implement one smoke case first. Continue through the suite, preserving skipped and failing cases in the report. Stop before release.

What happened

Willison reports a dependency-free JavaScript library passing thousands of compatibility tests; the session summary also lists skipped cases.

Reported by Simon Willison

How the agent did it

  1. Read the Python parser and compatibility tests and prepare an API specification.
  2. Implement a small working parser before expanding through the planned milestones.
  3. Run the compatibility suite repeatedly and commit progress, then add a browser playground.

Where a person stepped in

Willison reviewed the specification and smoke milestone, monitored commits and resumed the run after a usage limit.

A person approved each step before the agent continued.

More details

About this account

Codex worked through implementation milestones and test failures between a small number of follow-up instructions.

Firsthand technical write-up links code and tests. Compatibility is a reported outcome, not a guarantee of production suitability.

Credit: Simon Willison

Source published 15 Dec 2025

Source checked 14 Oct 2026

Tools and topics

Codex CLI · GPT-5.2 · html5lib-tests · Git · GitHub Actions

Code · Testing

Editorial completeness: 100/100. Measures documented fields, not agent performance or verified outcomes.

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