We calculate the action and the interaction Hamiltonians to all orders in perturbation theory in the model of single field inflation with a transient ultra slow-roll phase. Employing the formalism of effective field theory (EFT) of inflation, we obtain a compact nonperturbative expression for the interaction Hamiltonian in terms of the Goldstone field $\pi$ in the decoupling limit. In addition, we also present a nonlinear relation between $\pi$ and the curvature perturbations to all orders in perturbation theory. These are powerful results which enable us to calculate the cosmological correlators and loop corrections to any order in perturbation theory. As a nontrivial example, we calculate the $L$-loop corrections on long cosmic microwave background (CMB) scale perturbations in the ultraslow roll (USR) models which are used for primordial black holes (PBHs) formation. We show that the loop corrections scale like $(\mathrm{\Delta }N{\mathcal{P}}_{e}L{)}^L$ in which ${\mathcal{P}}_e$ is the peak of the power spectrum and $\mathrm{\Delta }N$ is the duration of the USR phase. This indicates that the loop corrections grow quickly out of perturbative control for large values of $L$. In the conventional USR setup for PBHs formation with $\mathrm{\Delta }N\simeq 2.5$, this happens at $L=4$.

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• Inflation