The part people don't think to worry about: who's allowed to hit the brakes
There's a third thing worth naming, because it's the one that surprises people. The instinct once you're worried about an agent doing the wrong thing is to add a watcher: something that observes the agent and can step in. That instinct is correct.
What people don't think through is that the watcher is now also a thing that can go wrong, and if it has unrestricted power to freeze or interrupt whatever it's watching, a single miscalibrated judgement from the watcher is now a new way for legitimate work to stop, not just a new way for bad work to get caught.
We looked at this specifically and found almost nobody in the category has thought about it as a design problem. Most systems have no watcher at all. The rare exception we found, buried inside one system's engineering, is a narrow detector that halts an agent when it notices obviously repetitive behaviour, nothing broader.
Nobody has a general answer for the harder version: a watcher that can genuinely catch subtle problems also needs genuinely serious authority to act on what it finds, and serious authority in the wrong hands, or the wrong calibration, is its own liability.
The direction we're taking, not yet built, is treating a watcher's authority as something that has to be explicitly bounded per watcher, not granted wholesale. A narrow, mechanical check, the kind that catches an agent obviously going in circles, can reasonably be trusted to pause things on its own.
A broader, more judgement-based watcher, the kind that's trying to notice something subtler, should be able to flag and ask, not freeze on its own authority. Different watchers, different power, decided deliberately rather than defaulted to either extreme.