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Rewrites are now good, contrary to the conventional wisdom from The Mythical Man-Month.
Thariq Shihipar argues that the old rule against rewrites no longer applies, because AI tools can distill a codebase's spec into a new implementation and force better test coverage. ✦ AI generated
Thariq Shihipar · Simon Willison's Weblog · 2026-07-21 · original ↗
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“What's a piece of conventional software engineering that was true a year ago that you don't think holds anymore in this new world?”
All the Mythical Man-Month stuff — never rewrite — I'm pro-rewriting now. If you have a good test suite — and I think the rewrite actually forces you to make sure you have a good test suite — but I think what people undercount is that a codebase is a spec, and maybe it's the only copy of the spec that you have, because no one knows every branching part of the codebase. You can take this as an artifact and distill it or create other versions of it.
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supports → A year-long, multi-engineer rewrite of Bun from Zig to Rust was completed in 11 days using 64 parallel AI agents and $165K in tokens.Jarred Sumner · The Pragmatic Engineersupports → AI coding models have become good enough since summer 2025 that resisting letting them write code directly no longer makes sense, reversing the author's prior stance.David Heinemeier Hansson · The Pragmatic Engineersupports → Having one AI model review another model's code work is a genuinely valuable practice rather than mere superstition.Simon Willison · Simon Willison's Weblogsupports → The way to offset the sense of grief or loss from AI taking over software engineering work is to be more ambitious about what you build.Thariq Shihipar · Simon Willison's Weblog