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Claude Code now bundles a Rust-rewritten Bun, ~10% faster startup on Linux

Bun creator Jarred Sumner ported the runtime to Rust, and Claude Code v2.1.181 and later ship it as an unreleased Bun v1.4.0 preview that starts about 10% faster on Linux. Simon Willison confirmed it by pulling 563 Rust source-file names out of the Claude binary, and any developer can install the same port today via bun upgrade --canary.

Source: simonwillison.net ↗

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Boring is good.

Jarred Sumner

Why this matters

  • → 10% faster startup improves developer experience across millions of Claude Code users
  • → Demonstrates production-scale Rust rewrites can ship invisibly, reducing maintenance burden
  • → Preview releases bundled in tools reach developers faster than formal channels
Invisible upgrade