GitHub just rewrote the Copilot agent runtime in Rust. Not a small migration either. ~430K lines of production TypeScript passed through the port, resulting in 800K+ lines of production Rust. And the wildest part? AI agents wrote most of it. The migration landed through 128 PRs over roughly 14.5 weeks, while Copilot continued shipping throughout the process. The goal wasn't simply Rust is faster. Rust gave the runtime - lower startup and runtime overhead - predictable resource usage - a native binary - C ABI interoperability - easier embedding across C#, TypeScript, Python, Rust, Go and Java - less dependency on Node.js/V8 GitHub also reported orders-of-magnitude performance improvements. But there were plenty of regressions. The team still had to debug correctness issues, lifecycle/state bugs, behavioural mismatches and host-boundary problems. That's the important lesson. AI didn't magically rewrite 800K lines of code correctly. It made a rewrite economically possible that previously would've required a large team and 1–2 years. The interesting future isn't AI replaces programmers. It's that projects which were previously too expensive to attempt are becoming feasible. Rust + AI coding agents is going to be a very interesting combination.