Claude Code Integrates Rust-Rewritten Bun, Boosting Linux Startup Speed by 10%

Claude Code version 2.1.181 has integrated the newly Rust-rewritten Bun JavaScript runtime, resulting in a 10% startup speed improvement on Linux. This integration was discovered by developer Simon Willison after analyzing the tool's executable files. This update demonstrates the immediate real-world performance gains of Bun's transition from Zig to Rust for developer tools. It also showcases the rapid adoption of the rewritten runtime by major AI-assisted development tools like Claude Code. The integration was confirmed by the presence of "Bun v1.4.0" and numerous ".rs" file extension strings within the Claude Code executable. The underlying Bun rewrite from Zig to Rust was reportedly completed in just 11 days using the Claude Fable 5 AI model.