Bun Rewritten in Rust Using AI Agents for Enhanced Memory Safety

Jarred Sumner has detailed the process of rewriting the Bun JavaScript runtime from Zig to Rust, a significant engineering feat that took considerably less time than initially anticipated. The project was driven by a need to address memory management issues, specifically use-after-free, double-free, and memory leaks, which are common sources of bugs. Rust's strong memory safety guarantees, enforced at compile time through features like RAII and the `Drop` trait, were identified as a solution to these problems. The existing Bun test suite, written in TypeScript, proved invaluable as a conformance suite, enabling automated porting of the codebase to Rust. This allowed for the use of AI agents, specifically models like Mythos/Fable from Anthropic, to assist in the rewrite. Sumner experimented with these agents, initially expecting limited success, but was surprised to see a high percentage of the test suite passing quickly. The AI-assisted rewrite involved extensive monitoring and iterative refinement of the agent's workflows, with Claude being prompted to edit the code generation loops. Reviewing over a million lines of AI-generated code required a robust strategy, including a comprehensive test suite, adversarial code review, and a focus on fixing the code generation process itself rather than manual bug fixes. The new Rust implementation of Bun has been integrated into Anthropic's Claude Code, with early results showing a 10% faster startup time on Linux, though overall user impact has been minimal. The project incurred significant computational costs, estimated at $165,000, utilizing billions of input and output tokens for the AI models.