> the interpreter that acts on those is a piece of classical code, and sits outside of the model.

Correct, but it's an LLM that's reasoning about what stream of interpretable tokens should be emitted. The interpreter can certainly apply some security measures around what's being asked of it (like ask for confirmation), but that can only go so far. Is the human in the loop always capable of understanding what's safe to execute? If not, should we pass it through another fallible LLM to help make that judgement call?

Some security measures can be handled in a purely deterministic manner. But not all of them, and that's the problem.

The mistake is in treating the LLM as just another deterministic, narrow computer program it can reason about. It's not. It's a "DWIM" system, and unless you can always precisely express what you mean - which you can't (not the very least because often people only realize what they meant after they get a result that's not it) - you have to treat the LLM as a human-like component. It's what it was designed to be anyway.