Saturn's Children

Saturn's Children. It's a neat idea for a story(humans have died off and robots have colonized the solar system) and it's pretty well written.

A novel that qualifies as hard SF in which every character is an AI? Saturn's Children by Charlie Stross. 100% not what you have in mind, though. (You'll enjoy it, though.) OTOH, I always find "Hard SF" a self-defeating category. I mean, anything with humans living on a different planet is pretty definitionally not Hard SF. Even Solar System stuff like The Expanse has troubles with economic plausibility. That leaves us with... well basically a couple of Vernor Vinge novels.