Article · 2026-07-19 · 4 moments

Claude Code uses Bun written in Rust now

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). Th ✦ AI generated

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GUEST Jarred Sumner HOST Simon Willison

Fact

A second strings/grep command against the Claude Code binary extracts a list of 563 Rust source filenames, including files like src/runtime/bake/dev_server/mod.rs and src/bundler/bundle_v2.rs, embedded in the compiled binary.

A second command extracting .rs file paths from the binary turns up 563 Rust source filenames, giving direct evidence that Bun's Rust codebase is compiled into the shipped tool.

transcript

Simon Willison: 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

extends · 1

Claim◆

Claude Code version 2.1.181 (released June 17th) and later versions run on the Rust port of Bun, which made startup about 10% faster on Linux, though almost nobody noticed the change.

Jarred Sumner states that Claude Code has been shipping with the Rust-rewritten version of Bun since v2.1.181, yielding a modest Linux startup speedup that went largely unnoticed.

transcript

Jarred Sumner: 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.

supports · 3

Data

Running strings on a local Claude Code binary and grepping for 'Bun v1' reveals the string 'Bun v1.4.0 (macOS arm64)', a version number newer than any publicly released Bun build.

The author ran a strings/grep command on their own Claude Code install and found a 'Bun v1.4.0' identifier, implying Anthropic is bundling a preview of an unreleased Bun version.

transcript

Simon Willison: 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.

Claim◆

The combined evidence confirms that Bun's Rust rewrite is genuinely running in production on millions of devices via Claude Code, quietly and without fanfare.

The author concludes that the strings evidence proves Bun-in-Rust is live in production at massive scale through Claude Code, echoing Sumner's 'boring is good' framing.

transcript

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

supports · 2

Highlight slides

Bun's Rust Rewrite: Live in Production✦ from: The combined evidence confirms that Bun's Rust rewrite is genuinely running in production on millions of devices via Claude Code, quietly and without fanfare.Claude Code Quietly Switched to Rust-Based Bun✦ from: Claude Code version 2.1.181 (released June 17th) and later versions run on the Rust port of Bun, which made startup about 10% faster on Linux, though almost nobody noticed the change.The Verdict: 'Boring Is Good'✦ from: The combined evidence confirms that Bun's Rust rewrite is genuinely running in production on millions of devices via Claude Code, quietly and without fanfare.Boring Is Good✦ from: Claude Code version 2.1.181 (released June 17th) and later versions run on the Rust port of Bun, which made startup about 10% faster on Linux, though almost nobody noticed the change.

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