Rewriting Bun in Rust | Bun Blog

Why & how we rewrote Bun from Zig to Rust

Disclosure: Bun was acquired by Anthropic in December 2025. I and others on the Bun team work at Anthropic. I used a pre-release version of Claude Fable 5 for much of the Rust rewrite. Bun started as a line-for-line port of esbuild's JavaScript & TypeScript transpiler from Go to Zig. I wrote my first line of Zig on April 16, 2021 . I bet on Zig after seeing the single-page Zig Language Reference on Hacker News and getting really excited about the low-level control and care for performance. From the start, Bun's scope was massive: JavaScript, TypeScript, and CSS transpiler, minifier, and bundle

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