Claude Code, Anthropic's coding tool, uses Bun as its runtime under the hood, and Bun has been rewritten in Rust. Bun is a runtime built as a faster alternative to Node.js, originally written in the Zig . The change means Claude Code's underlying execution engine has shifted to this new Rust-based version of Bun.
Key points
Claude Code's runtime is built on Bun
Bun itself has been rewritten from Zig into Rust
Bun aims to be a faster drop-in alternative to Node.js