Claude Code uses Bun written in Rust now
Claude Code v2.1.181+ ships Bun's Rust rewrite in production across millions of devices
“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.”
Claude Code quietly shipped with a pre-release Rust-rewritten version of Bun (v1.4.0) as its embedded JavaScript runtime, delivering 10% faster startup on Linux. Simon Willison confirmed this by inspecting the Claude binary for embedded Rust source filenames and the Bun version string. The migration affected millions of devices with no user-facing disruption, serving as a real-world validation of Bun's Rust rewrite at scale.