Bun Rewrites Its Runtime from Zig to Rust Using Coding Agents — 1M+ Lines, 11 Days, ~$165K in Tokens

Summary

Jarred Sumner and the Bun team described a complete rewrite of the Bun runtime from Zig to Rust, carried out largely by AI coding agents — a project that would normally violate the "never rewrite from scratch" rule but became tractable with modern agentic engineering. The stated motivation was memory-safety reliability: a large share of Bun's bugs were use-after-free, double-free, and forgotten-free errors that Rust's compiler eliminates by construction. The effort added over a million lines and