What I find particularly useful is secrets never leave the local machine of the user and are never saved. The safest thing of course is agent can never see and or use the secret but then running tests and experiments could be hard.
In general I find that anything you put in the harness is likely to be owned, compromised or at risk. I just don't think the security primitives are widely there for these tools no matter what we're told. Agents will find a way to do something if they're given access in the harness and to the LLM.