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.

Historically, rewrites are a terrible idea. Excluding comments, Bun is 535,496 lines of Zig. A rewrite in another language would take a small team of engineers a full year. It would mean freezing bugfixes, security fixes or feature development for that time. The least risky approach to getting something shippable would be a mechanical port from Zig to Rust, with the minimal number of behavioral changes, using the exact same test suite we already use for testing Bun. By hand, I think this would've taken three engineers with full context on the codebase about a year, during which time we wouldn't be able to improve Node.js compatibility, fix bugs, fix security issues or implement new features. We never would've done that.

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