Claude Code Moves to Bun — Written in Rust: A New Era of Performance
Anthropic's Claude Code CLI has migrated to the Bun runtime — which is built on Zig and Rust. A significant technical evolution for AI developer tools.
Claude Code's Migration to Bun — A Performance Revolution
Anthropic's Claude Code CLI has undergone a significant technical evolution — it has migrated to the Bun runtime, which is itself built on Zig and Rust. This change dramatically improves Claude Code's performance and reliability.
What Happened?
On July 19, 2026, Simon Willison published an observation that Claude Code's latest version (v2.1.181) now runs on the Bun runtime. Bun is a fast JavaScript/TypeScript runtime designed as a drop-in replacement for Node.js.
The Story of Bun: From Zig to Rust
Bun was originally created by Jarred Sumner in Zig programming language. The decision to rewrite Bun in Rust was driven by better memory management, ecosystem integration, faster iteration, and access to Rust's mature package ecosystem.
Why Rust Matters
Rust offers C++-level performance with memory safety guarantees. The entire Bun rewrite took 11 days with AI assistance using Claude Code itself.
What This Means for Claude Code Users
Users experience approximately 10% faster startup times, reduced memory footprint, and improvements in dependency handling and module resolution.
The Bigger Picture: Convergence of Ecosystems
This migration is part of a larger trend where developer tools increasingly leverage Rust for performance-critical components while maintaining JavaScript/TypeScript interfaces.
Conclusion
The Claude Code migration to Bun represents a significant technical milestone. It demonstrates how next-generation AI tools adopt cutting-edge technology to deliver better performance.