I know how to write a test that verifies other tests. But I can do that already, I'm wondering what Beagle would add.

I also know that if the meta-test is writable to the agent, it will change it if it feels like it wants to get rid of some other test. Even if it can't change the meta-test, it can hollow out existing tests to make them pass trivially.

I don't think nondeterminism is the problem. The problem is following rules: If I tell the agent not to change tests, it can conveniently "forget" about this. It doesn't much matter if it forgets deterministically. The problem is that it can forget at all.