  The effective action includes all quantum corrections arising in a given
quantum field theory. Thus it serves as a powerful generating functional from
which quantum-corrected scattering amplitudes can be constructed via tree-level
computations. In this work we use this framework for studying gravity-mediated
two-to-two scattering processes involving scalars and photons as external
particles. We construct a minimal basis of interaction monomials capturing all
contributions to these processes. This classification goes beyond the
expansions used in effective field theory since it retains the most general
momentum dependence in the propagators and couplings. In this way, we derive
the most general scattering amplitudes compatible with a relativistic quantum
field theory. Comparing to tree-level scattering in general relativity, we
identify the differential cross sections which are generated by the non-trivial
momentum dependence of the interaction vertices.
