Bun's creator Jarred Sumner rewrote the entire Bun JavaScript runtime from Zig to Rust in just 11 days using Anthropic's Fable AI agent tool, at a cost of $165K in API tokens. The process involved 64 parallel Claude agents working across 1,448 files (535K lines of code), guided by a detailed 600-line porting guide created upfront. Key steps included prep work with Claude, parallel agent execution, adversarial review of each commit, and iterative compiler error fixing. The migration succeeded largely because Bun had a robust test suite. The takeaway: AI-assisted rewrites are now feasible for well-tested codebases, compressing what would have been a 1-2 year manual effort into under two weeks, though it requires a motivated engineer with deep codebase knowledge, a comprehensive test suite, and willingness to invest in token costs.
