Claude Code confirmed using Bun’s Rust port in production

Simon Willison reports that Claude Code versions 2.1.181 and later include a Rust-based port of Bun, following a claim by Bun’s author Jarred Sumner. Sumner stated that these versions ship with the Rust port and observed a 10% startup speed improvement on Linux, describing the change as intentionally unremarkable: 'Boring is good.'

Willison verified the presence of the Rust-based Bun runtime by inspecting his local Claude Code binary. Running strings against the binary returned Bun v1.4.0 (macOS arm64), a version not yet publicly released by Bun as of May 12, 2025. This suggests Claude Code is shipping an unreleased preview of Bun’s Rust port.

Further inspection of the binary with a regex matching Rust source file paths surfaced 563 filenames, including src/runtime/bake/dev_server/mod.rs and src/runtime/bake/production.rs, indicating the Rust source tree is embedded in the shipped binary.

Willison concludes that the Rust port of Bun is running in production across millions of devices, based on the evidence from his binary inspection and Sumner’s report.