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Claude Code uses Bun written in Rust now

Claude Code just ditched Node. Now it's running on Bun—written in Rust. Here's why that matters for your stack.

Reporting by Simon Willison 1 min brief
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The KeyNews take

Why it matters

Anthropic is quietly upgrading Claude Code's runtime infrastructure to Bun (a faster, Rust-based JavaScript runtime), signaling a shift toward performance-optimized tooling for AI-assisted development. This affects how developers experience AI coding agents and suggests infrastructure-level improvements in the agentic coding space.

The key facts

7 to know

  1. Claude Code now uses Bun as its runtime

  2. Bun is written in Rust

  3. Migration from Node.js implied

  4. Published July 19, 2026

  5. Claude Code now uses Bun as runtime

  6. Published July 18, 2026

  7. Infrastructure optimization for AI dev tools

Go to the source

Simon Willisonsimonwillison.net

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