Claude Code uses Bun written in Rust now

Simon Willison reports that Claude Code—Anthropic's AI-assisted development tool—has been running a Rust version of Bun, the JavaScript runtime, since version two point one point one eighty-one. Willison investigated the binary and found five hundred sixty-three Rust source files inside, along with Bun version one point four point zero—a preview release that hasn't landed officially yet. Linux users saw a ten percent startup speedup. Jarred Sumner, Bun's creator, called it boring engineering. And that's exactly the point—this quiet upgrade has been running across millions of devices unnoticed.

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