---
title: "Claude Code Now Ships with the Rust-Ported Bun"
source: "Simon Willison 博客（Atom）"
source_tier: "t2"
url: "https://simonwillison.net/2026/Jul/19/claude-code-in-bun-in-rust/#atom-everything"
permalink: "https://feed.astralor.com/items/itm_dbd57b2637da08ce64d9881a"
published: "2026-07-19T03:54:09.000Z"
score: 75
channel: "ai"
tags: ["Anthropic","Bun","Claude Code","Rust","startup performance"]
---

# Claude Code Now Ships with the Rust-Ported Bun

## AI commentary

> This article caught my eye because it goes beyond features to uncover a core technology switch in Claude Code. For developers focused on AI tool performance optimization, it offers a rare inside look—the fact that a Rust rewrite could be deployed without user disruption shows Anthropic's solid engineering in compatibility and performance. If you're considering a Rust port of your own runtime, study Willison's forensic approach.

## At a glance

As of Claude Code v2.1.181 (released June 17th), the tool ships with the Rust port of Bun (v1.4.0, not yet publicly released). Author Simon Willison discovered evidence by scanning the binary with strings, finding both the Bun version number and 563 .rs source file paths. Linux startup improved by ~10%, with negligible impact on other platforms. Bun creator Jarred Sumner commented, 'Boring is good,' highlighting a seamless migration.

## Key points

- Claude Code v2.1.181+ uses the Rust port of Bun
- Evidence: strings command shows Bun v1.4.0 (unreleased) and 563 Rust file paths
- Linux startup ~10% faster; other platforms unchanged
- Bun creator calls the boring migration a positive sign

## Full text

In Rewriting 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 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. strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs' This outputs a list of 563 filenames, 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". Tags: bun, rust, anthropic, claude-code, jarred-sumner

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Verify against the original before citing; curated by Astral Feed: https://feed.astralor.com/items/itm_dbd57b2637da08ce64d9881a