The test suite is the interface
The Bun Zig-to-Rust rewrite cost $165K in tokens and touched over a million lines. Everyone focuses on the scale. That’s not what I find interesting.
What makes this work isn’t that an agent can write code. It’s that the TypeScript test suite served as a language-independent executable spec. A million assertions that don’t care whether the runtime underneath is Zig or Rust. Without that oracle, a +1M-line PR is unreviewable — no amount of adversarial review saves you when you can’t tell correct from plausible.
The job shifts from reviewing code to reviewing the process that generates it. You don’t fix bugs in the output. You fix the loop, re-run, and let the test suite tell you if you’re done.