Bun's creator details the Zig to Rust rewrite: agentic engineering, bug fatigue, and technical trade-offs
Jarred Sumner details his decision to rewrite the Bun JavaScript runtime from Zig to Rust, explaining the technical challenges, agentic engineering workflows used during the rewrite, and the motivations behind the switch. He praises Zig for getting Bun this far but cites a growing bugfix list and fatigue as key reasons for moving to Rust.
Key quotes
Our bugfix list felt bad and I was tired
Jarred Sumner has been promising this blog post (since May 9th) about his Zig to Rust rewrite of Bun for significantly longer than it took him to finish the rewrite.
This is a detailed description of an extremely sophisticated piece of agentic engineering, featuring dynamic workflows, trial runs, adversarial review and all sorts of other interesting tricks.
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