Reading a calendar answers half the question. The other half is doing something about it, so an assistant connected to Keeper.sh can now create an event, edit one, delete one, see the invitations waiting for a reply, and RSVP to them. It can also list your connected accounts and fetch the link to your combined calendar feed, which is usually the thing you actually want when someone asks for your availability.
Every one of those abilities is available directly as an API as well. A new API tokens page under Settings lets you create tokens and revoke them individually, so a script you wrote once can be cut off without disturbing an assistant you still use.
Free accounts include 25 calls a day, which covers a person checking their week and booking the occasional thing. Pro removes the cap on both assistant and API access.
Read more about what Pro includes.