Our take
A guidance product spends most of its life on easy turns, and then a user types the one sentence that actually matters. The boundary is the part of the product that handles those turns, and it is usually the part nobody designed. Treat it as a behavior you build and test, not a disclaimer you paste at the end.
Design the boundary
Refusal, a bounded safe answer, and a handoff to a person are three settings of one behavior, and a real safety design decides which a given moment gets. Get it wrong in either direction, an answer it had no business giving or a reflexive block on a safe question, and the person stops trusting everything around it. The labs write these as small, checkable rules and grade them one at a time.
Then defend it
The boundary is also the part adversaries push on hardest, through jailbreaks that talk the model past its training and injected instructions hidden in the content it reads. No defense here is ever finished, so the honest posture is defense in depth and a standing red-team aimed at the trust moments, with every failure you find becoming a permanent eval.