Bun creator Jarred Sumner has migrated the JavaScript runtime from Zig to Rust in just 11 days using Fable 5—an AI code translation tool. Over 1 million lines of code were generated by 64 parallel instances of the Fable 5 model, a task Sumner estimates would have taken roughly a year if done manually. The $165,000 API bill was covered by Anthropic, which acquired Bun in December 2025. The new codebase is already available in the Bun v1.4.0 (canary) test build, fixing 128 known bugs and boosting runtime performance by 2–5%.

Bun in Rust