Claude Code Now Runs on Bun — Rewritten in Rust, 10% Faster Startup

Anthropic's Claude Code has adopted the Rust rewrite of the Bun runtime, shipping in v2.1.181 with 10% faster startup and improved performance. The Bun creator confirmed the Rust port is now in production.

Anthropic's Claude Code — the terminal-based AI coding agent — has quietly adopted one of the most significant runtime upgrades in the JavaScript ecosystem. Starting with version 2.1.181, released June 17, Claude Code ships with the Rust rewrite of Bun, the all-in-one JavaScript runtime created by Jarred Sumner.

Sumner confirmed the switch in a post about the Bun rewrite, noting that the Rust port delivers approximately 10% faster startup times compared to the original Zig implementation. For a tool like Claude Code that launches frequently during development sessions, even small startup improvements compound into a noticeably smoother experience.

Simon Willison highlighted the change, observing that this makes Claude Code one of the first high-profile production deployments of the Rust-based Bun. The news reached the top of Hacker News with over 570 points, reflecting developer enthusiasm for both the technical achievement and the performance implications.

The shift signals growing confidence in Rust as a systems language for JavaScript tooling — a space historically dominated by C++, Zig, and Go. With Claude Code serving thousands of developers daily, the Rust-powered Bun now has a major production proving ground.

Sources: Simon Willison | Claude Code Changelog