Under the duality between the two-dimensional XY model and an integer-valued height function, the BKT transition is expected to correspond to the disappearance of a corner in the surface tension; in the delocalised phase, its zero-slope curvature should determine the Gaussian free field prefactor. We prove that the XY mass equals the right derivative at zero slope of the dual height function's free energy, with a uniform quadratic error bound near the origin. Thus the massive phase is exactly the corner regime, while in the BKT phase the surface tension is quadratically bounded at zero slope. The proof combines Kadanoff--Ceva duality, Ginibre's inequality, and a pushing lemma (an estimate of Russo--Seymour--Welsh type) for the cable height function.