Bun has completely rewritten its core engine in Rust, abandoning Zig after the previous language caused persistent memory errors and crashes. Developer Jarred Sumner credits Anthropic‘s Fable 5 model for generating the majority of the new codebase during an eleven-day period. Approximately sixty-four instances of the model ran simultaneously to produce over a million lines of code, resulting in a bill of around $165,000. This expenditure would have required a human team roughly a year to match. The new release, version 1.4.0, is currently available as a canary build. It resolves one hundred and twenty-eight bugs and improves performance by two to five percent compared to the earlier version. Sumner noted that the switch to Rust catches many bugs at compile time, offering the reliability Zig could not provide. The project proceeded without financial strain because Bun and Sumner’s team were acquired by Anthropic in December 2025.
This migration demonstrates how large language models can accelerate complex systems engineering while prioritising stability over raw speed. The high cost of generation was offset by the acquisition deal, removing commercial pressure to minimise expenses immediately. The shift also signals a preference for Rust’s memory safety guarantees in high-performance JavaScript tooling.
- Over one million lines of code generated in eleven days
- Performance increase of two to five percent
- One hundred and twenty-eight bugs fixed in the new release
