In diversity actions, federal courts apply state substantive law and federal procedural law. Under Fed. R. Civ. P. 81(c)(1), “The [federal] rules apply to a civil action after it has been removed from a state court.” Federal procedural rules also apply pending a resolution of the district court's jurisdiction.
The test for fraudulent joinder is whether a reasonable basis exists for predicting that the plaintiff’s claims against the non-diverse defendant could succeed under state law. It makes little sense to measure the state-law viability of such claims, which were originally filed in state court, by federal pleading standards.