How is security handled in a situation like this? (I'll end up reading up on it). But I mean things like how does the API let a specific session with a specific user only surface information for that specific user from the available tooling?
The current framework that solves this the best right now is FiPA.
Establish identity the same way we always have, AS defines the challenges, the client (voice agent in this case) goes and gets them. OAuth from there!