A camshaft phaser includes a housing with a harmonic gear drive unit disposed therein. The harmonic gear drive unit includes a circular spline and a dynamic spline, a flexspline disposed radially within the circular spline and the dynamic spline, a wave generator disposed radially within the flexspline, and a rotational actuator connectable to the wave generator. One of the circular spline and the dynamic spline is fixed to the housing. A hub is rotatably disposed radially within the housing and attachable to the camshaft and fixed to the other of the circular spline and the dynamic spline. A first arcuate input stop member is rotatable with one of the circular spline and the dynamic spline and is received within a first arcuate output opening defined by at least a first arcuate output stop member rotatable with the other of the circular spline and the dynamic spline.