  The next decade will feature an abundance of novel cosmological data, while
many fundamental questions about inflation remain. Given this, there is ample
need for maximally efficient calculations, especially in non-standard scenarios
for the early Universe. In inflationary cosmology, observables are computed
within the framework of in-in perturbation theory. Weinberg introduced a
now-widely used re-organization of perturbation theory for in-in calculations.
There is a subtle difference in the $i\epsilon$ prescriptions of Weinberg's
perturbation series with traditional in-in, which could interfere with the
projection onto the interacting vacuum. In this work, we show that a small
adjustment to Weinberg's perturbation series yields agreement with standard
in-in at every order of perturbation theory for commonly studied spins and
masses in de Sitter spacetime. We then generalize the result to a large class
of cosmological spacetimes, including slow roll spacetimes.
