An axiomatic characterization of Steenrod's cup-$i$ products

We show that any natural construction of cup-$i$ products on the normalized cochains of simplicial sets, parameterized by the minimal free resolution of the trivial ${\mathbb{F}}_2[{\mathbb{S}}_2]$-module, is isomorphic—not just homotopic—to Steenrod’s original construction if it is non-zero, irreducible, and free. We show that these three conditions are independent, and use them to prove that all cup-$i$ constructions in the literature represent the same isomorphism class.