Claude Code v2.1.181 and later ships with a Rust-rewritten version of Bun, confirmed. Jarred Sumner announced the change on the Bun blog, claiming 10% faster startup on Linux. The rollout has been silent by design.
Two shell commands verify this independently. Running `strings ~/.local/bin/claude | grep -m1 'Bun v1'` returns `Bun v1.4.0 (macOS arm64)`, a version that does not exist in the public GitHub releases, where the latest is v1.3.14 from May 12th. Running a second grep for `.rs` file paths surfaces 563 Rust source filenames embedded in the binary, including `src/bundler/bundle_v2.rs` and `src/runtime/bake/dev_server/mod.rs`.
The original post is worth reading because the methodology is the story. Simon Willison used only `strings` and `grep` against a local binary to independently validate a production infrastructure claim made by the framework's own creator. No source code access, no insider knowledge. The embedded file paths are the receipt.
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