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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.

A key enabling condition for the rewrite was that Bun already had a language-independent TypeScript test suite, which agents could use to verify correctness while porting code from Zig to Rust. ✦ AI generated

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

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A crucial enabling factor for the rewrite was that the Bun test suite was written in TypeScript, which meant it could act as a conformance suite. This allowed an agent harness to automate much of the initial port from Bun to Rust, initially as an experiment to try out an earlier version of the model we now have access to as Mythos/Fable.

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✳ Bun ✳ Mythos/Fable ✳ TypeScript

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What matters in agentic coding is not whether you end up with passing tests, but how you got there and how structured the process is — agentic coding tends to produce a working 'spaghetti monster' of code that no one has actually read or understood.Tim Scarfe · Machine Learning Street Talk · conf 60%

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gives example → 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.Simon Willison · Simon Willison's Weblog