Best of Simon Willison — December 2025

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   Article

   Simon Willison·23w

   JustHTML is a fascinating example of vibe engineering in action

   JustHTML is a pure Python HTML5 parser that passes all 9,200+ browser vendor tests and achieves 100% test coverage. The library was built over several months using AI coding agents (Claude Sonnet, Gemini Pro, Claude Opus) in VS Code, but with extensive human engineering oversight. The developer established the API design, integrated comprehensive test suites, built custom profilers and fuzzers, and made all architectural decisions while letting the AI handle code implementation. This represents "vibe engineering"—using AI agents professionally with proper code review, testing, and engineering practices—rather than "vibe coding" which produces unvetted prototypes. The project demonstrates how experienced engineers can leverage AI as a typing assistant while maintaining responsibility for design, quality, and architectural decisions.

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   Article

   Simon Willison·23w

   Your job is to deliver code you have proven to work

   Software engineers must deliver proven, working code rather than untested contributions. This requires both manual testing (seeing the code work yourself, documenting steps, testing edge cases) and automated testing (bundling tests with changes). With AI coding agents like Claude Code, developers should train these tools to prove their changes work through testing before submission. The human developer remains accountable for ensuring code quality and providing evidence that changes function correctly.

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