Effective field theory (EFT) approaches are widely used at the LHC, such that it is important to study their validity, and ease of matching to specific new physics models. In this paper, we consider an extension of the SM in which a top quark couples to a new heavy scalar. We find the dimension six operators generated by this theory at low energy, and match the EFT to the full theory up to NLO precision in the simplified model coupling. We then examine the range of validity of the EFT description in top pair production, finding excellent validity even if the scalar mass is only slightly above LHC energies, provided NLO corrections are included. In the absence of the latter, the LO EFT overestimates kinematic distributions, such that over-optimistic constraints on BSM contributions are obtained. We next examine the constraints on the EFT and full models that are expected to be obtained from both top pair and four top production at the LHC, finding for low scalar masses that both processes show similar exclusion power. However, for larger masses, estimated LHC uncertainties push constraints into the non-perturbative regime, where the full model is difficult to analyse, and thus not perturbatively matchable to the EFT. This highlights the necessity to improve uncertainties of SM hypotheses in top final states.
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