Claude Code has quietly shipped on a Rust rewrite of Bun since v2.1.181
Jarred Sumner says Claude Code v2.1.181 (released June 17) and later builds run on a Rust port of Bun. Startup on Linux got about 10% faster; almost no one noticed any other difference.
Simon Willison confirmed it on his own install. Running strings on the claude binary turned up Bun v1.4.0, while the latest public Bun release on GitHub is v1.3.14 from May 12. In other words, Claude Code ships a runtime version that hasn’t been released yet.
A second pass found 563 files with a .rs extension, including src/runtime/bake/production.rs and src/bundler/bundle_v2.rs. Sumner has said he moved Bun from Zig to Rust in 11 days using roughly 50 parallel Claude Code workflows.
Up next
Anthropic’s six-step playbook for migrating code with Claude Code
Porting Bun to Rust through Claude Code cut memory use from 6,745 MB to 609 across a 2,000-build test, shrank the binary 19%, and lifted speed 2 to 5%. Anthropic’s Jarred frames the migration as a six-step loop judged by the compiler, tests, and a line-by-line diff.
Related stories
- Perf win of the day: Claude Code CLI now uses 2x less CPU at p99. Bun’s garbage collector was running on a fixed tim…
- Zig’s creator calls the rewritten Bun “untested garbage”
- Claude Code 2.1.318: login expiry warning, runner log change
- Intent-lens spells out Claude Code calls before they run
- Codedict cuts Claude Code tokens 63%, but cost only 26%
- claude-files-pane shows Claude Code’s file reads live
Comments
No comments yet. Be the first.
Join the conversation
Sign in to leave a comment. It appears after moderation.