Claude Code has quietly shipped on a Rust rewrite of Bun since v2.1.181
Jarred Sumner says Claude Code v2.1.181 (released June 17) and later builds run on a Rust port of Bun. Startup on Linux got about 10% faster; almost no one noticed any other difference.
Simon Willison confirmed it on his own install. Running strings on the claude binary turned up Bun v1.4.0, while the latest public Bun release on GitHub is v1.3.14 from May 12. In other words, Claude Code ships a runtime version that hasn't been released yet.
A second pass found 563 files with a .rs extension, including src/runtime/bake/production.rs and src/bundler/bundle_v2.rs. Sumner has said he moved Bun from Zig to Rust in 11 days using roughly 50 parallel Claude Code workflows.
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