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Claude Code runs on Bun — and Bun is now written in Rust

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Simon Willison proved it on his own install: since version 2.1.181, Claude Code uses the Rust port of Bun. Startup got 10% faster — and almost nobody noticed. That's exactly the point.

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Sometimes the best technical news is the kind nobody notices. Simon Willison wrote up one of those on July 19 — and because it lives under the hood of Claude Code, it’s worth a look.

What’s going on

Bun’s creator Jarred Sumner announced that Bun has been rewritten in Rust. Buried in the announcement was this line: Claude Code v2.1.181 and later (released June 17) use the Rust port of Bun. Startup got about 10% faster on Linux — “and otherwise, barely anyone noticed. Boring is good.”

Willison didn’t just take that on faith — he went looking. He poked at his own Claude Code install and found two commands that convinced him:

strings ~/.local/bin/claude | grep -m1 'Bun v1'

For him that prints Bun v1.4.0 (macOS arm64) — a version that doesn’t officially exist on GitHub yet (the latest release there is v1.3.14 from May 12). So Anthropic is shipping a preview of an unreleased Bun. The second command surfaces a list of 563 Rust filenames baked into the binary — src/bundler/bundle_v2.rs and friends.

The takeaway: Bun-in-Rust is running in production across millions of devices. No announcement, no migration drama.

My take

I love stories like this, because they show how software actually gets better today — not with a big bang, but with the quiet swap of a component you never see. Your Claude Code starts faster, you type the same commands, and the whole switch from a JavaScript runtime to Rust sailed right past you.

That’s exactly the kind of maturity that turns a tool from a prototype into an everyday appliance. “Boring is good” isn’t understatement — it’s a quality promise. If you’re building something developers reach for daily, you can’t afford drama under the hood. Rust mostly buys predictability here: fewer surprises around memory and performance.

And the little footnote — that Claude Code bundles an unreleased Bun version — might be the most interesting part. Anthropic is testing infrastructure on its broad user base before it even officially exists. A little bold — but that’s how you get the ten percent nobody notices.

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