  Positivity bounds represent nontrivial limitations on effective field
theories (EFTs) if those EFTs are to be completed into a Lorentz-invariant,
causal, local, and unitary framework. While such positivity bounds have been
applied in a wide array of physical contexts to obtain useful constraints,
their application to inflationary EFTs is subtle since Lorentz invariance is
spontaneously broken during cosmic inflation. One path forward is to employ a
$\textit{Breit parameterization}$ to ensure a crossing-symmetric and analytic
S-matrix in theories with broken boosts. We extend this approach to a theory
with multiple fields, and uncover a fundamental obstruction that arises unless
all fields obey a dispersion relation that is approximately lightlike. We then
apply the formalism to various classes of inflationary EFTs, with and without
isocurvature perturbations, and employ this parameterization to derive new
positivity bounds on such EFTs. For multifield inflation, we also consider
bounds originating from the generalized optical theorem and demonstrate how
these can give rise to stronger constraints on EFTs compared to constraints
from traditional elastic positivity bounds alone. We compute various shapes of
non-Gaussianity (NG), involving both adiabatic and isocurvature perturbations,
and show how the observational parameter space controlling the strength of NG
can be constrained by our bounds.
