Do you write the test yourself or get the agent to do it?

No OP but I also guide LLMs with TDD and it's a mixture of LLMs write tests for happy paths and I write tests for edge cases.

Also when I use LLM to fix a bug, I tell it to write a test to prevent regression of the bug at the end of the session, after the bug is fixed.

I get the agent to do it generally. (I realize this seems incestuous, but its fairly easy to validate the tests are sensible as you add features, because the biggest risk is regressions as the AI does something dumb later.)