We study the full stable pair theory -with descendents -of the Calabi-Yau 3-fold X = K S , where S is a surface with a smooth canonical divisor C.By both C * -localisation and cosection localisation we reduce to stable pairs supported on thickenings of C indexed by partitions. We show that only strict partitions contribute, and give a complete calculation for length-1 partitions. The result is a surprisingly simple closed product formula for these "vertical" thickenings.This gives all contributions for the curve classes [C] and 2[C] (and those which are not an integer multiple of the canonical class). Here the result verifies, via the descendent-MNOP correspondence, a conjecture of Maulik-Pandharipande, as well as various results about the Gromov-Witten theory of S and spin Hurwitz numbers. 1 We emphasise that in this paper we are concerned with the full stable pair and Gromov-Witten invariants of X, not their reduced cousins computed in [KT1, KT2].2 In fact all we require, by the deformation invariance of stable pair and Gromov-Witten invariants, is that some deformation of S should have such a divisor.