We show that supersymmetry can be used to compute the BCFT one-point function coefficients for chiral primary operators, in 4d \( \mathcal{N} \) = 2 SCFTs with \( \frac{1}{2} \) -BPS boundary conditions. The main ingredient is the hemisphere partition function, with the boundary condition on the equatorial S3. A supersymmetric Ward identity relates derivatives with respect to the chiral coupling constants to the insertion of the primaries at the pole of the hemisphere. Exact results for the one-point functions can be then obtained in terms of the localization matrix model. We discuss in detail the example of the super Maxwell theory in the bulk, interacting with 3d \( \mathcal{N} \) = 2 SCFTs on the boundary. In particular we derive the action of the SL(2,ℤ) duality on the one-point functions.
