Claude Code uses Bun written in Rust now

Simon Willison · Simon Willison · 2026-07-19

Simon Willison inspects the Claude Code binary and confirms it ships with a Rust-rewritten version of Bun (v1.4.0, not yet publicly released), providing reproducible string-inspection commands as evidence.

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Extraction

Topics: claude-codebun-runtimerustdeveloper-tools

Claims

Claude Code v2.1.181 and later ship with the Rust port of Bun as their embedded JavaScript runtime.
The Bun version bundled in Claude Code (v1.4.0) is newer than the latest public Bun release (v1.3.14 from May 12th), meaning Anthropic is shipping a pre-release Bun build.
String inspection of the Claude Code binary reveals 563 Rust source file paths, confirming the Rust rewrite is running in production across millions of devices.
The Rust rewrite produced only a 10% startup speed improvement on Linux and went largely unnoticed by end users.

Key quotes

Claude Code v2.1.181 (released June 17th) and later use the Rust port of Bun. Startup got 10% faster on Linux but otherwise, barely anyone noticed. Boring is good.

It looks like Bun in Rust is indeed being run in production across millions of different devices. Like Jarred said, 'Boring is good'.