I think 'hard SF' is usually afforded a couple of 'gimmes' in the sense of rule breaking. I imagine most people would place Rendezvous With Rama, for instance, in this class despite (spoilers, hah) the reveal of Rama's voodoo propulsion mechanism. Niven's books, while fun, push this way beyond what I'd normally concede to be 'hard' but then maybe I'm out of touch.
Also just wanted to say I really enjoyed Accelerando, and the Laundry Files series is amazing so far (although I don't know how far I'll get if it keeps getting darker!) Thanks for all your good work.
I mean, Rendezvous with Rama tries its best to place Rama's reactionless system in the context of currently known physics, using distortion of the fabric of space as its cause. But even including that, the important part is that the protagonists don't get access to the same technology; for example, the Endeavor didn't get to cheat its way out of its hyperbolic escape orbit (it had to be rescued by suicide tanker vessels). So while it's technically "soft" in this aspect it doesn't affect the plot.
Also just wanted to say I really enjoyed Accelerando, and the Laundry Files series is amazing so far (although I don't know how far I'll get if it keeps getting darker!) Thanks for all your good work.