Federal common law governs the effect of a federal court’s judgment on federal law claims (Taylor v. Sturgell (2008) 553 U.S. 880, 891) whereas state law governs the res judicata effect of a federal court’s judgment on state law claims when exercising diversity jurisdiction (Semtek Internat. Inc. v. Lockheed Martin Corp. (2001) 531 U.S. 497, 507). It is unclear which law applies when a federal court’s judgment disposes of state law claims over which the federal court exercised pendent jurisdiction. Acknowledging a conflict between Hardy v. America’s Best Home Loans (2014) 232 Cal.App.4th 795 (state law) and Guerrero v. Department of Corrections & Rehabilitation (2018) 28 Cal.App.5th 1091 (federal law) on this question, this decision follows Guerrero, holding that federal common law governs the res judicata effect of a federal court’s judgment on state law claims heard in its pendent jurisdiction.