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That bill looks wasteful until you see what Jarred Sumner’s team pulled off: a production-grade language migration most teams could not finish in a year, at any price #
Jarred Sumner, the creator of the Bun JavaScript runtime, rewrote its entire Zig codebase to Rust in 11 days, using 64 parallel AI agents. (About 20% of Bun was already C and C++, and that part stayed put; the Zig core is what moved.) The result shipped, passed its own test suite, and went green on CI. That sentence alone is worth pausing on, because most projects that get pitched at this scope don’t finish. This one did. Sumner wrote up the whole thing in detail on the Bun blog, and the numbers below come from that writeup and the reporting around it.
Bun isn’t a hobby project. It ships 22 million downloads a month and sits underneath tools people use daily, including Claude Code, OpenCode, and hosting platforms like Vercel, Railway, and DigitalOcean. A rewrite of its core runtime is not a weekend refactor. It’s the type of project that normally gets scoped in quarters, staffed with a small team of engineers who already know the codebase cold, and still runs long.