Bun ditches Zig for Rust with help from Claude Fable 5, writes over a million lines of code in 11 days

The JavaScript tool Bun has been fully rewritten from Zig to Rust, and Anthropic's Fable 5 did most of the work. Developer Jarred Sumner says the switch came down to reliability. Zig kept producing memory errors and crashes that were hard to fix for good. Rust catches many of those bugs at compile time.

Sumner used a pre-release version of Claude Fable 5 for the project. About 64 Claude instances ran in parallel for 11 days, writing over a million lines of code. The API bill came to roughly $165,000. A human team would have needed about a year, according to Sumner. The new version, Bun v1.4.0, is available as a canary release. It fixes 128 bugs and runs about 2 to 5 percent faster. Sumner didn't have to worry about the hefty price tag. Bun and his team were acquired by Anthropic in December 2025.