Claude Code Now Runs on Bun Written in Rust

Anthropic's Claude Code CLI has quietly switched to using Bun written in Rust, marking a notable shift in how AI coding tools handle their runtime dependencies.

Bun creator Jarred Sumner confirmed that Claude Code v2.1.181 and later versions use the Rust port of Bun as their embedded JavaScript runtime. The change delivers 10% faster startup on Linux, though Sumner noted that barely anyone noticed the switch.

The Evidence

Simon Willison independently verified the change by inspecting the Claude Code binary. Running strings on the executable revealed Bun v1.4.0 embedded, alongside 563 Rust source files - clear evidence that the Rust port is running in production across millions of devices.

The Rust version of Bun has been released as a canary build. Users can run 'bun upgrade --canary' to install it. The version number 1.4.0 has been sitting in Bun's package.json since a commit on May 17th, but has not yet made it into a tagged release.

Why It Matters

The switch represents a broader trend of AI coding tools adopting modern JavaScript runtimes for performance and reliability. Bun's Rust implementation brings substantial performance improvements without changing the developer experience. For founders building on Claude Code, this means faster startup times and a more responsive coding assistant.