Completeness of Cartesian theories

If a Cartesian sequent fails to factor its antecedent subobject $A\hookrightarrow C$ through its consequent, the covariant representable functor ${\mathcal{C}}_{\mathbb{T}}(A,-)$ is a set-valued countermodel. It preserves finite limits, and the element named by the antecedent inclusion belongs to the antecedent but cannot belong to the consequent. Hence validity in every set model implies Cartesian derivability.