JavaScript runtime Bun runs Claude Fable 5 for 11 days to migrate from Zig to Rust. How was Claude used?

Bun creator Jarred Sumner used Claude Code and Claude Fable 5 to port roughly 535,000 lines of Zig to Rust.

The migration reportedly took about 11 days and combined workflow automation, lifetime analysis, adversarial code review, and test-driven fixes.

The Rust version was said to reduce leaks, shrink binary size, and improve performance.

The case shows how AI agents can speed up large systems-language migrations while improving memory safety and runtime stability.