Claude Code uses Bun written in Rust now

# Claude Code uses Bun written in Rust now Source: [https://simonwillison.net/2026/Jul/19/claude-code-in-bun-in-rust/](https://simonwillison.net/2026/Jul/19/claude-code-in-bun-in-rust/) 19th July 2026 In[Rewriting Bun in Rust](https://bun.com/blog/bun-in-rust)Jarred Sumner made the following claim: > 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: ``` strings ~/.local/bin/claude | grep -m1 'Bun v1' ``` For me this outputs`Bun v1\.4\.0 \(macOS arm64\)`\. The most recent release of[Bun on GitHub](https://github.com/oven-sh/bun/releases)is currently[v1\.3\.14](https://github.com/oven-sh/bun/releases/tag/bun-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\. \(**Update**: The Rust version*has*been released as[Bun canary](https://bun.com/docs/installation#canary-builds)\- running`bun upgrade \-\-canary`will install[this release](https://github.com/oven-sh/bun/releases/tag/canary)\.\) ``` strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs' ``` This outputs a list of[563 filenames](https://gist.github.com/simonw/c92fb0f67b114ac26e3b95a09ddccfdc), starting with these: ``` src/runtime/bake/dev_server/mod.rs src/runtime/bake/production.rs src/bundler/bundle_v2.rs ``` 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](https://twitter.com/ajanraj25/status/2078825794701242697): ``` cat > /tmp/bun-version.ts <<'EOF' console.log("embedded bun:", Bun.version); process.exit(0); EOF BUN_OPTIONS="--preload=/tmp/bun-version.ts" claude --version ``` This outputs`1\.4\.0`for me\. Here's[the commit from May 17th](https://github.com/oven-sh/bun/commit/b18bf6d1d0a92238f240bfd125f0e3b3461b9243#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519)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`\.