# Simon Willison’s Weblog [Simon Willison’s Weblog](/) [Subscribe](/about/#subscribe) [Learn more](https://fandf.co/4gCMW1I) 19th July 2026 In Rewriting Bun in Rust Jarred Sumner made the following claim: [Rewriting Bun in Rust](https://bun.com/blog/bun-in-rust) Claude Code v2.1.181 (released

Simon Willison’s Weblog

Learn more

19th July 2026

In Rewriting Bun in Rust Jarred Sumner made the following claim:

Rewriting Bun in Rust

Claude Code v2.1.181 (released June 17th) and later use the Rust port of Bun. Startup got 10% faster on Linux but otherwise, barely anyone noticed. Boring is good.

I decided to have a poke at my own Claude Code installation to see if I could find evidence that it was using Bun written in Rust.

I found these two commands convincing:

For me this outputs Bun v1.4.0 (macOS arm64). The most recent release of Bun on GitHub is currently v1.3.14 from May 12th, so that v1.4.0 version number in Claude supports them shipping a preview of a not-yet-released Bun version.

Bun on GitHub

v1.3.14

(Update: The Rust version has been released as Bun canary - running bun upgrade --canary will install this release.)

Bun canary

this release

This outputs a list of 563 filenames, starting with these:

563 filenames

It looks like Bun in Rust is indeed being run in production across millions of different devices. Like Jarred said, "Boring is good".

Update: Here's a neat trick from Ajan Raj:

from Ajan Raj

This outputs 1.4.0 for me.

Here's the commit from May 17th that updated the version in package.json to 1.4.0. That version hasn't been changed since then, but also hasn't yet made it into a tagged release outside of canary.

the commit from May 17th

19th July 2026

Claude Code uses Bun written in Rust now

Simon Willison’s Weblog

Recent articles

Monthly briefing