Claude Code uses Bun written in Rust now

This article investigates the claim that Claude Code v2.1.181 and later versions use a Rust port of Bun, as stated by Jarred Sumner. The author examines their own Claude Code installation, finding evidence through command-line tools that the embedded Bun version is v1.4.0 (a preview release) and listing 563 Rust source files, confirming Bun in Rust is running in production. The article includes a trick to verify the embedded Bun version and notes a 10% startup improvement on Linux. It is relevant to IT/Technology as it covers software engineering, runtime internals, and production deployment of a rewritten tool.