Simon Willison • 7/19/2026

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 string analysis that the binary contains Bun v1.4.0 (a preview version not yet publicly released) and references to Rust source files (.rs extensions). A trick using a Bun script confirms the embedded Bun version. The article concludes that Bun in Rust is running in production across millions of devices, highlighting the 'boring is good' philosophy of stable, incremental improvements.

0 comments

#performance #rust #javascript runtime