  In this paper we establish an almost optimal well-posedness and regularity
theory for the Klein-Gordon-Schr\"odinger system on the half line. In
particular we prove local-in-time well-posedness for rough initial data in
Sobolev spaces of negative indices. Our results are consistent with the sharp
well-posedness results that exist in the full line case and in this sense
appear to be sharp. Finally we prove a global well-posedness result by
combining the $L^2$ conservation law of the Schr\"odinger part with a careful
iteration of the rough wave part in lower order Sobolev norms.
