Bun, the popular JavaScript runtime, has been completely rewritten from Zig to Rust—and AI did the heavy lifting. Developer Jarred Sumner decided to switch languages because Zig kept causing memory errors and crashes that were tough to permanently fix. Rust, on the other hand, catches many of those bugs at compile time, making the codebase more reliable. The rewrite was executed using a pre-release version of Anthropic’s Claude Fable 5. Sumner ran around 64 Claude instances in parallel for 11 days, generating over a million lines of code. The API bill? Roughly $165,000. For context, he says a human team would have needed about a year. The new version, Bun v1.4.0, is now available as a canary release. It fixes 128 bugs and runs 2 to 5 percent faster. Why it matters: This project shows that AI can now handle massive-scale rewrites in days instead of months, dramatically changing how core infrastructure tools are built and maintained.