Claude Code uses Bun written in Rust now

Anthropic's Claude Code v2.1.181 now uses a Rust port of Bun, achieving 10% faster startup on Linux. Evidence from the binary shows Bun v1.4.0 and Rust source files, confirming the switch to a production-ready Rust-based runtime.

In Rewriting Bun in Rust https://bun.com/blog/bun-in-rust Jarred Sumner made the following claim: Claude Code v2.1.181 released June 17th and later use the Rust port of Bun. Startup got 10% faster on Linux but otherwise, barely anyone noticed. Boring is good. I decided to have a poke at my own Claude Code installation to see if I could find evidence that it was using Bun written in Rust. I found these two commands convincing: strings ~/.local/bin/claude | grep -m1 'Bun v1' For me this outputs Bun v1.4.0 macOS arm64 . The most recent release of Bun on GitHub https://github.com/oven-sh/bun/releases is currently v1.3.14 https://github.com/oven-sh/bun/releases/tag/bun-v1.3.14 from May 12th, so that v1.4.0 version number in Claude supports them shipping a preview of a not-yet-released Bun version. strings ~/.local/bin/claude | grep -Eo 'src/ :alnum: ./- +\.rs' This outputs a list of 563 filenames https://gist.github.com/simonw/c92fb0f67b114ac26e3b95a09ddccfdc , starting with these: src/runtime/bake/dev server/mod.rs src/runtime/bake/production.rs src/bundler/bundle v2.rs It looks like Bun in Rust is indeed being run in production across millions of different devices. Like Jarred said, "Boring is good". Tags: bun https://simonwillison.net/tags/bun , rust https://simonwillison.net/tags/rust , anthropic https://simonwillison.net/tags/anthropic , claude-code https://simonwillison.net/tags/claude-code , jarred-sumner https://simonwillison.net/tags/jarred-sumner