To be sure, we had hidden tests that validate whether agent implemented the task appropriately. And this is something we paid a lot of attention to (pass rate, in our paper.)

What we didn't do (stupid oversight on my part) was to ensure that there are no regressions in the remaining tests in the repo (unrelated to the current task). In practice, when using Sonnet 4.6 IRL, I don't see a lot of regressions because often the agent runs the test before calling it done. But it could have gone either way. We don't know.