Bun rewrites 1M+ lines from Zig to Rust in 11 days with Claude Fable 5

11天Claude Fable 5写超100万行代码：Rust重构JavaScript运行时Bun

Jarred Sumner ran 64 Claude Fable 5 instances in parallel for 11 days to rewrite the entire Bun JavaScript runtime from Zig to Rust, producing over 1 million lines of code. API costs hit $165K, but Bun was acquired by Anthropic in December 2025 so the bill isn't a concern. The main driver was reliability: Zig's memory errors and crashes were hard to fix, while Rust catches many of them at compile time. Bun v1.4.0 shipped as a canary release with 128 bugs fixed and a 2–5% speedup. Sumner estimates a human team would have needed a year.

Why it matters: First public large-scale AI-assisted rewrite case after Anthropic's acquisition of Bun: 64 parallel instances over 11 days produced 1M+ lines of Rust, with $165K in API fees absorbed by the acquisition. Numbers are concrete, source is first-person, details are operational — al...