Bun's creator Jarred Sumner details how the entire Bun JavaScript runtime was rewritten from Zig to Rust in 11 days using Claude Code with 64 parallel AI agents. The motivation was systematic memory safety: Zig's lack of RAII-style Drop semantics led to persistent use-after-free, double-free, and memory leak bugs. Rather than enforcing style guides, the team leveraged Rust's borrow checker and Drop trait to make these bugs compile-time errors. The rewrite used dynamic workflows with adversarial review agents, a PORTING.md guide, and a LIFETIMES.tsv file to ensure behavioral fidelity. The result: Bun v1.4.0 fixes 128 bugs, reduces memory usage dramatically (e.g. 6.7 GB → 609 MB for 2,000 builds), shrinks binary size by ~20%, and improves HTTP throughput by 2–5%. The process cost ~$165,000 in API tokens and took one engineer 11 days instead of an estimated 3 engineers for a year.
