Claude Code now ships with Rust-rewritten Bun v1.4.0
Original: Claude Code uses Bun written in Rust now
Why This Matters
Claude Code's quiet adoption of pre-release Rust-rewritten Bun signals a major runtime infrastructure shift at scale.
As of version 2.1.181 (released June 17, 2026), Claude Code bundles a Rust port of the Bun JavaScript runtime (v1.4.0), ahead of its official public release. Linux startup speed improved by 10%, with minimal disruption reported across millions of devices.
Bun creator Jarred Sumner confirmed that Claude Code v2.1.181 and later versions ship with a Rust-rewritten port of the Bun JavaScript runtime. Simon Willison verified this by inspecting his local Claude Code binary: the command `strings ~/.local/bin/claude | grep -m1 'Bun v1'` returned `Bun v1.4.0 (macOS arm64)`, a version ahead of the latest public Bun release (v1.3.14, dated May 12). A second command extracting `.rs` filenames from the binary returned 563 Rust source file paths, providing further evidence of the Rust port running in production. Sumner noted that Linux startup time improved by roughly 10%, and that the transition went largely unnoticed — a result he described as 'boring is good.' An additional technique shared by developer Ajan Raj — using `BUN_OPTIONS` to preload a TypeScript file — also confirmed Bun v1.4.0 embedded in the binary. The v1.4.0 version string has been present in the package.json since a May 17 commit but has not yet appeared in a tagged release outside of Bun's canary channel. Running `bun upgrade --canary` will install this preview release.