Bun

“Claude Code runs on Bun, a JavaScript runtime built on Zig... Its large, mature test suite was the verification mechanism that made the run possible.”

“"A Bun engineer had thrown this exact task at every new model generation as an informal benchmark, and prior models failed outright."”

“Jared on the team was like, okay, let's just rewrite it. Maybe the model can do this... And starting with Claude's newer models, the model started to be able to do it... He had the model rewrite it from Zig to Rust. It was one prompt. It was a dynamic workflow. And it ran for 11 days.”

“Bun's team used agent loops to rewrite roughly 750,000 lines from Zig to Rust in 11 days, with 99.8% of tests passing.”