Simon Willison: Bun's Zig-to-Rust Rewrite Was Largely Done by Claude Agents

simonwillison.net·high signal

Simon Willison surfaces Jarred Sumner's writeup of rewriting Bun from Zig to Rust — PR #30412 (merged May 14) added ~1M lines across 2,188 files, reached 99.8% test compatibility on Linux x64, and shrank the binary 3–8MB. Much of the translation was performed by Claude agents (a pre-release Fable 5) running six days on high-memory nodes, producing 6,755 commits after Anthropic's acquisition of Bun. A landmark case study in AI-assisted large-scale language migration.