Ask a vendor this in your first call. The answer tells you more than any case study will.
A good answer describes a confidence threshold and a clean escalation. A bad answer is some version of "our AI handles everything", which means either they have not run one in production or they are hoping you will not check.
No agent handles everything. The ones claiming to are the ones that strand people in loops.
The two ways it goes wrong
It guesses. This is the failure everyone worries about, and it is genuinely fixable. An agent answering from a model's general knowledge hallucinates somewhere in the 15 to 27% range. One that retrieves from a verified knowledge base before responding drops to under 1.5%.
That difference is not a better model. It is whether the thing is grounded in your actual content, and it is the single most important technical requirement in the whole category.
It hedges. Less discussed and more common. The agent answers correctly, then adds "please confirm with a member of our team before proceeding". The customer reads that as uncertainty, asks for a human, and you have an escalation on a question the agent answered perfectly well.
We hit this ourselves. Resolution rate sat stuck for weeks while the transcripts looked fine, because the agent was being politely unsure at the end of correct answers.
What good looks like instead
Three things, and they are all architectural rather than conversational.
An explicit threshold. Above it, the agent answers directly and without hedging. Below it, it stops trying and hands over. The mushy middle, where it half-answers and half-defers, is where trust goes.
Routing that means something. Billing goes to billing, faults go to faults. Not one undifferentiated queue where a specialist question waits behind twenty password resets.
Context carried across. The human picks up with the customer verified, the issue captured and the transcript attached. If the customer has to start again, the handover failed no matter how good the first half was.
Done properly, most people do not notice the conversation changed hands.
The bit worth being blunt about
A bad handover is worse than no agent. "I'll connect you to someone who can help", then five minutes of silence, then a person asking for the account number again, is precisely the experience that taught a generation of customers to type "agent" the moment a chat window opens.
You are not just failing that conversation. You are confirming a suspicion they already had, and it carries into every future interaction.
What to ask before you sign
Ask to see the threshold. Ask what percentage of conversations cross it. Ask what the human sees when a handover lands, and ask to look at that screen.
A partner who runs agents in production can show you all three. One who only has a demo will start talking about the technology instead, and that pivot is your answer.
