Claude Code uses Bun written in Rust now

Claude Code v2.1.181 and later versions now use Bun written in Rust instead of the original JavaScript version, with the change delivering a 10% speed improvement on Linux while remaining largely unnoticed by users. Simon Willison verified this by examining his Claude Code installation and finding evidence of Rust source files and Bun v1.4.0 (a preview release not yet publicly available) running in production across millions of devices.