We consider the S-matrix in Type IIB/IIA string theory, and compare it to correlators of the stress tensor multiplet in various holographic CFTs. On flat space, the leading small momentum corrections to these S-matrices are protected and known at finite string coupling. At finite string coupling, Type IIB onAdS_5 x S_5 is dual to 4d N=4 SYM at large N and finite complexified coupling tau, while Type IIA on AdS_4 x S^7/Z_k is dual to 3d ABJM with Chern-Simons level k at large N and fixed N/k^5. In each case, we compute the CFT correlator to several orders in 1/N using supersymmetric localization, and find that it matches the known S-matrix in the flat space limit, which is the first non-perturbative (in string coupling) check of AdS/CFT. In the Type IIB case, the 1/N coefficients are modular functions of tau under S-duality, which receive infinite instanton corrections. In the Type IIA case, the 1/N coefficients receive no instanton corrections, and interpolate from M-theory at small k (large string coupling) to Type IIA at large k (weak string coupling).
