Claim◆Article
Claude Code version 2.1.181 (released June 17th) and later versions run on the Rust port of Bun, which made startup about 10% faster on Linux, though almost nobody noticed the change.
Jarred Sumner states that Claude Code has been shipping with the Rust-rewritten version of Bun since v2.1.181, yielding a modest Linux startup speedup that went largely unnoticed. ✦ AI generated
Jarred Sumner · Simon Willison's Weblog · 2026-07-19 · original ↗
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.
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- ·v2.1.181 (June 17th) and later use Bun's Rust port
- ·Startup ~10% faster on Linux
- ·Barely anyone noticed the change
- ·Jarred Sumner: change shipped with minimal fanfare
- ·Performance gain without user-facing disruption
- ·Signals a stable, low-drama migration
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Evidence · 3
The combined evidence confirms that Bun's Rust rewrite is genuinely running in production on millions of devices via Claude Code, quietly and without fanfare.Simon Willison · Simon Willison's Weblog · conf 75%Running strings on a local Claude Code binary and grepping for 'Bun v1' reveals the string 'Bun v1.4.0 (macOS arm64)', a version number newer than any publicly released Bun build.Simon Willison · Simon Willison's Weblog · conf 65%A second strings/grep command against the Claude Code binary extracts a list of 563 Rust source filenames, including files like src/runtime/bake/dev_server/mod.rs and src/bundler/bundle_v2.rs, embedded in the compiled binary.Simon Willison · Simon Willison's Weblog · conf 60%