Claude Code Now Powered by Bun (Written in Rust)

Originally posted by Simon Willison on July 19th, 2026

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The Core Claim

Recently, Jarred Sumner asserted that starting with Claude Code v2.1.181 (which debuted on June 17th), the tool began utilizing a version of Bun rewritten in Rust.

"Claude Code v2.1.181 and later use the Rust port of Bun." — Jarred Sumner

Regarding the impact of this change, the results are modest:

• Linux users: Experienced a startup speed increase of $\approx 10\%$.
• General users: Noticeable differences Barely any perceptible change.

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Investigating the Evidence

To verify if my own installation of Claude Code was indeed running the Rust-based Bun, I performed a few technical checks.

Verification Checklist

• Check binary strings for versioning
• Search for Rust source file references
• Execute a runtime version check

Method 1: Version String Analysis

I ran the following command to peek into the binary:

strings ~/.local/bin/claude | grep -m1 'Bun v1'

Result: Bun v1.4.0 (macOS arm64)

This is significant because of the version discrepancy shown below:

Note: The Rust version has since been released as a Bun canary build; you can get it via bun upgrade --canary.

Method 2: Hunting for Rust Artifacts

I searched the binary for any file paths ending in .rs (the extension for Rust files):

strings ~/.local/bin/claude | grep -Eo 'src/[[:alnum:]_./-]+\.rs'

This returned 563 distinct filenames. Some examples include:

• src/runtime/bake/dev_server/mod.rs
• src/runtime/bake/production.rs
• src/bundler/bundle_v2.rs

This strongly suggests that the Rust implementation of Bun is currently live in production across millions of devices.

Method 3: The Runtime Trick

Thanks to a clever tip from Ajan Raj, I was able to force Claude to reveal its embedded version by preloading a TypeScript file.

The Setup:

// Save this as /tmp/bun-version.ts
cat /tmp/bun-version.ts 'EOF'
console.log("embedded bun:", Bun.version);
process.exit(0);
EOF

The Execution:

BUN_OPTIONS="--preload=/tmp/bun-version.ts" claude --version

Output: 1.4.0

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Technical Summary & Timeline

The transition seems to have been codified in a commit dated May 17th, where the package.json was updated to version 1.4.0. While this version hasn't seen a formal tagged release outside of the canary channel, it is clearly the engine driving Claude Code.

Logic Flow of Verification

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Related Reading:

• Kimi K3 and the pelican benchmark (July 16, 2026)
• The GPT-5.6 family: Luna, Terra, Sol (July 9, 2026)
• sqlite-utils 4.0 migrations (July 7, 2026)