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Abstract Stress‐based postseismic deformation modeling including afterslip and viscoelastic relaxation usually assumes the coseismic slip distribution and the associated stress perturbation as known. However, that assumption biases the postseismic modeling results by the assumptions that underlie the coseismic models. Importantly, this misses an opportunity to iteratively constrain the coseismic slip model with postseismic observations. We used a broad set of seismic and geodetic data to create multiple coseismic slip models that only differ in the down‐dip extent of the rupture plane and fit the coseismic observations for the July 29, Mw 8.2 Chignik earthquake equally well. We then evaluated the quality of those coseismic slip models based on how well each of them predicts postseismic GNSS displacements using a stress‐driven afterslip model. We find that coseismic slip models that generate afterslip too far down‐dip systematically fail to predict postseismic deformation. We find that the postseismic observations are best predicted by a narrower coseismic slip model that terminates abruptly at its deepest extent. The model predictions improve further if stress‐driven afterslip is combined with a superimposed viscoelastic relaxation response of a 50 km thick elastic lithosphere for the overriding plate and an elastic cold nose to the mantle wedge.