I'm a big fan of prototyping functionality with no UX input at all, so that the engineers have full authority to look at all the moving parts and figure out the

I'm a big fan of prototyping functionality with no UX input at all, so that the engineers have full authority to look at all the moving parts and figure out the best way to make them work...without any design constraints. At the point we have a functional prototype, go through it with UX so they can understand all of the pieces before they do the design. Sometimes it leads to minor improvements on what the engineers already came up with. Other times it leads to an overhaul. But as long as all of the core functionality is built first, iterating on the design will just be moving a few things around.