11 Days Claude Fable 5 Crazy Writing Over 1 Million Lines of Code: Rust Refactoring JavaScript Runtime Bun

IT House reported on July 11 that developer Jarred Sumner published a blog post on July 8, announcing that with the help of the Claude Fable 5 model, it took 11 days and cost $165,000 to reconstruct the JavaScript runtime Bun and completely rewrite it from Zig to the Rust language. Sumner said that the main reason for this reconstruction is to improve reliability. Zig often suffers from memory errors and crashes, and it is difficult to completely repair. Rust catches many of these errors at compile time. Sumner said that during the refactoring process, 64 Claude instances ran in parallel for 11 days and more than 1 million lines of code were written, while refactoring by a human team would have taken a year. The cost of the API is approximately US$165,000 (IT House Note: The current exchange rate is approximately RMB 1.119 million), but Bun and his team were acquired by Anthropic in December 2025, so Sumner does not have to worry about high costs. Bun v1.4.0 was released as a canary version, fixing 128 bugs and running about 2% to 5% faster.