Simon Willison — Claude Code v2.1.181 ships a Rust-built Bun v1.4.0

What is it?

Simon Willison's July 19 post shows that the Claude Code CLI (from v2.1.181 onward) now runs on Bun v1.4.0, a preview build that Bun's team is rewriting from Zig into Rust. Anthropic ships that unreleased Bun inside every Claude Code install without flagging the change to users.

How does it work?

The proof is two shell one-liners against the Claude Code binary. `strings ~/.local/bin/claude | grep -m1 'Bun v1'` prints "Bun v1.4.0 (macOS arm64)" — a version ahead of Bun's public v1.3.14 tag. A second `grep -Eo 'src/[[:alnum:]_./-]+\.rs'` pulls 563 Rust source filenames out of the same binary, matching Bun's in-progress Rust tree (`src/bundler/bundle_v2.rs`, `src/runtime/bake/dev_server/mod.rs`, etc.).

Why does it matter?

This is the first public evidence that the Rust port of Bun is already running Anthropic's coding agent for millions of installs. Anthropic gets a faster, statically-linked runtime; the Bun team gets battle-testing on real workloads before their own 1.4 tag; users see a ~10% Linux startup speedup with no announcement. It's a datapoint on how quickly major AI vendors are absorbing pre-release infrastructure.

Who is it for?

Claude Code users, runtime engineers, Bun contributors

Try it

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

Tags

• claude-code
• bun
• rust
• runtime
• anthropic
• simon-willison
• coding-agent
• developer-tools