Jarred Sumner, creator of the Bun JavaScript runtime, announced that he rewrote Bun from Zig to Rust in 11 days using multiple Claude AI agents. Bun is a JavaScript suite that includes a runtime, package manager, bundler and test runner. Anthropic acquired Bun in December 2025 and built its core state machine on it. The original Bun code was written in Zig, a language favored for its performance and low-level control. Sumner said the migration was driven by growing numbers of bugs reported by users, including a bug in the bundler that generated source maps despite being told not to, according to NodeSource. He estimated the work cost about $165,000 in API fees. Using roughly 50 parallel Claude Code workflows, the team generated more than a million lines of Rust code at a peak rate of 1,300 lines per minute, he reported. The Rust version passed all of Bun’s test suite, which contains more than one million assertions, without skipping any test. Sumner said the rewrite eliminated the need for a small engineering team to spend a year on a manual port, allowing continued development and bug fixes. Andrew Kelley, the creator of Zig, expressed concern that the rewrite reflected poor programming practices rather than language shortcomings, as noted in his blog post titled My Thoughts on the Bun Rust Rewrite. He said the move highlighted differing project philosophies. Kelley added that Bun’s rapid feature rollout led to accumulated technical debt and that the Zig community had rejected AI‑generated contributions, citing quality concerns. Mitchell Hashimoto, co‑founder of HashiCorp, commented on X that the speed achieved was unlikely for a salaried engineer. The rewrite demonstrates that large‑scale code migrations can be completed quickly with AI assistance, but critics have raised concerns about the maintainability of AI‑generated code.