Claude Code Now Runs on Bun Rewritten in Rust: Startup 10% Faster

Starting with version 2.1.181, released on June 17, 2026, Claude Code runs on a version of the Bun runtime rewritten from Zig to Rust. Bun creator Jarred Sumner announced the switch, and on July 19, 2026, developer Simon Willison independently confirmed it by picking apart the Claude Code binary. Startup on Linux is about 10% faster.

How Rust was found inside Claude Code

Simon Willison confirmed the switch by running the `strings` command on the Claude Code binary with a filter for the string "Bun v1". The output was `Bun v1.4.0 (macOS arm64)`. A second command, which searched for paths of the form `src/*.rs`, returned a list of 563 file names with the `.rs` extension, ranging from `src/runtime/bake/dev_server/mod.rs` to `src/bundler/bundle_v2.rs`. This is direct evidence that the runtime's Rust code is compiled into the binary.

• Claude Code v2.1.181 — released on June 17, 2026, the first version on the Rust port of Bun
• Embedded runtime — Bun v1.4.0; stable Bun releases currently top out at v1.3.14 (May 12, 2026)
• The `strings` command found 563 file names with the `.rs` extension
• Startup speedup on Linux — about 10%
• The Rust version is already publicly available as a canary build (`bun upgrade --canary`)

Why Claude Code ships Bun 1.4.0, which doesn't exist yet

Claude Code contains Bun v1.4.0, a version that is not yet in Bun's stable releases. The latest tag on GitHub is v1.3.14 from May 12, 2026, while the 1.4.0 number landed in `package.json` back in a commit from May 17, 2026, and has not changed since. In effect, Anthropic is shipping a pre-release build of the runtime in a production tool used by millions of developers.

You can check the embedded version without `strings`, too. Developer Ajan Raj suggested a trick: preload a script via the `BUN_OPTIONS` variable and run `claude --version`. The command prints `1.4.0`, the same runtime version.

"Boring is good" — what Bun's creator says

The transition went so smoothly that almost nobody noticed. That is exactly the point Bun's creator stresses: the main result of the Rust rewrite is not flashy features but stability and a slightly faster startup.

"Claude Code v2.1.181 (released June 17) and later versions use the Rust port of Bun.

Startup on Linux is 10% faster, but otherwise almost nobody noticed. Boring is good," — Jarred Sumner, creator of Bun, in the blog post "Rewriting Bun in Rust."

Simon Willison agrees with the assessment: in his words, "Bun on Rust really is running in production on millions of different devices."

What this means

Anthropic is using Claude Code as a proving ground to run the new Rust version of Bun in real production before its stable release. Users won't notice: the CLI simply starts a bit faster. For the Bun ecosystem, it is a battle test of the Rust port across a huge installed base.

Frequently asked questions

What is Bun?

Bun is a fast JavaScript runtime and an alternative to Node.js, and it is what Claude Code runs on. Bun was originally written in Zig; its core has now been rewritten in Rust.

How can I check which version of

Bun is embedded in Claude Code?

Run `strings ~/.local/bin/claude | grep -m1 'Bun v1'`; the output will show `Bun v1.4.0`. An alternative from Ajan Raj: run `claude --version` with the `BUN_OPTIONS` variable preloading a script that prints `Bun.version`; the result is the same, `1.4.0`.

Has Claude Code become faster?

Yes, but only slightly: according to Jarred Sumner, startup on Linux is about 10% faster. On other platforms, the difference is barely noticeable to the user.