Claude Code Has Been Running Rust-Bun Since June, Nobody Noticed
Claude Code v2.1.181 and later use a Rust port of Bun for about 10% faster startup on Linux, confirmed by Simon Willison via a binary string search.
What it is
A discovery from Simon Willison confirming a claim in Bun creator Jarred Sumner's post about rewriting Bun in Rust: Claude Code has quietly shipped on the new Rust-based Bun runtime since v2.1.181 (June 17th).
What it does
Instead of the original Bun (written largely in Zig), Claude Code's bundled runtime now runs on the Rust port, and Willison verified it locally by running `strings ~/.local/bin/claude | grep -m1 'Bun v1'` against his own install.
Why it matters
It is a real-world case of swapping a core runtime's implementation language under a widely used CLI tool with no user-visible regressions, just a quiet 10% startup speedup on Linux. Boring, successful infrastructure migrations rarely get told this clearly.
How to use it
Run `strings $(which claude) | grep -m1 'Bun v1'` on your own Claude Code binary to check which Bun build you're running, as described in Simon Willison's post.
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