  We determine the off-shell symmetry algebra and representations of Type IIB
superstring theory on $AdS_3\times S^3 \times T^4$ with mixed R-R and NS-NS
three-form flux. We use these to derive the non-perturbative worldsheet S
matrix of fundamental excitations of the superstring theory. Our analysis
includes both massive and massless modes and shows how turning on mixed
three-form flux results in an integrable deformation of the S matrix of the
pure R-R theory.
