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Coding agents powered by today's frontier models have overturned the old rule that you should never stop the world and rewrite a large piece of software from the ground up.

The author argues that Joel Spolsky's classic warning against full rewrites no longer fully applies now that frontier-model coding agents exist. ✦ AI generated

Simon Willison · Simon Willison's Weblog · 2026-07-08 · original ↗

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Everyone knows you should never stop the world and rewrite a large piece of software from the ground up. Joel Spolsky highlighted that in Things You Should Never Do, Part I back in April 2000! Coding agents powered by today's frontier models change that equation.

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✳ Bun ◉ Joel Spolsky

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Mechanism · 1

Coding agents have lowered both the effort of building a working automation and the cost of failed attempts, so the psychological burden of future maintenance or having to rewrite the code no longer discourages people from doing it.Simon Willison · Simon Willison's Weblog · conf 70%

In practice · 1

Bun's TypeScript-based test suite could double as a conformance suite, which is what allowed an agent harness to automate most of the initial Zig-to-Rust port using an early version of the Mythos/Fable model.Simon Willison · Simon Willison's Weblog · conf 60%

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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 Engineer