  Weyl semimetals are 3D condensed matter systems characterized by a degenerate
Fermi surface, consisting of a pair of `Weyl nodes'. Correspondingly, in the
infrared limit, these systems behave effectively as Weyl fermions in $3+1$
dimensions. We consider a class of interacting 3D lattice models for Weyl
semimetals and prove that the quadratic response of the quasi-particle flow
between the Weyl nodes is universal, that is, independent of the interaction
strength and form. Universality is the counterpart of the Adler-Bardeen
non-renormalization property of the chiral anomaly for the infrared emergent
description, which is proved here in the presence of a lattice and at a
non-perturbative level. Our proof relies on constructive bounds for the
Euclidean ground state correlations combined with lattice Ward Identities, and
it is valid arbitrarily close to the critical point where the Weyl points merge
and the relativistic description breaks down.
