If I understood correctly, the author had the agent perform manual mutation testing (write code, write passing tests, manually change the code to see some tests fail, then revert the change), rather than automated mutation testing. Why not use a mutation-testing framework and consider the build failed if a certain percentage of mutations are not killed?

The article claims that the agents didn’t actually use TDD or mutation testing. While it’s possible for an agent to ignore the TDD procedure even when instructed to follow it, it can’t ignore a build failure.