brief products
Claude Code quietly runs on Rust-built Bun
A teardown found Claude Code v2.1.181 swapped its JavaScript internals for a Rust-based Bun runtime in June, for a 10% faster Linux startup.
A teardown by Simon Willison found that Claude Code v2.1.181, released back in June, quietly switched its runtime to a Rust-compiled build of Bun (embedding a preview 1.4.0 build, 563 Rust source files in total), cutting Linux startup time by about 10%. Bun creator Jarred Sumner confirmed the change was built for reliability across millions of devices. No developer-facing behavior changed.
sources 1 cited
1 simonwillison.net Claude Code uses Bun written in Rust now