> how do they, for example, have a tool call that gives the customer a discount, without it getting used when it shouldn’t?

One possibility is that the company and the user both have AI agents, and that these are able to negotiate with each other. Then I talk to my local customized agent, which knows my preferences, and it explains the required context if the company’s agent does something that won’t make sense to me. But the company’s agent is still privy to the details required to offer discounts, say.