Calculating the action and the interaction Hamiltonian at higher orders in cosmological perturbation theory is a cumbersome task. We employ the formalism of effective field theory of inflation in the decoupling limit for single-field ultraslow-roll (USR) inflation and obtain a nonperturbative Hamiltonian in terms of the Goldstone field $\pi$. To complete the dictionary, a nonlinear relation between the curvature perturbations and $\pi$ is presented. Using these results, we compute higher-order loop corrections in USR models with a sharp transition to the attractor phase, relevant for primordial black holes formation. It is shown that in the idealized picture in which the transition from the USR phase to SR phase is instantaneous and sharp, the loop corrections on long cosmic microwave background scales increase rapidly with the number of loops $L$ and the setup may go out of perturbative control.

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• Inflation