We would need dev confirmation to be sure, but I think the AI always has, at a minimum, "scouted planet" intel on all planets (what stationary things you have on each of your planets, and where they are), and strength counts of any system they have a thing or adjacent too. Beyond that, it is hard to tell. For example, I don't think the AI always knows mobile unit composition and poaitioning for a planet unless they are on it, even if they know the total strength by just being adjacent. (One important exception, they know where exactly fallen spire "what you need to escort" stuff is at all times during their respective stages in the mission.)