Sage v. Wyncoot was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of mandamus to a federal court. The case arose when the plaintiff, Sage, sought to compel the defendant, Wyncoot, to appear in a state court to answer a complaint. Wyncoot argued that the state court lacked the authority to issue a writ of mandamus to a federal court. The Supreme Court held that the state court did not have the authority to issue a writ of mandamus to a federal court. The Court reasoned that the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of exclusive jurisdiction. The Court further reasoned that the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of limited jurisdiction. The Court concluded that the state court lacked the authority to issue a writ of mandamus to a federal court. The Court held that the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of exclusive jurisdiction and the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of limited jurisdiction. The Court also held that the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of limited jurisdiction and the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a court of exclusive jurisdiction.

In Sage v. Wyncoot, the Supreme Court was tasked with determining whether a contract between two parties that had been partially performed could be enforced by one of the parties against the other. The majority opinion held that it could not, as there were no mutual obligations remaining to be fulfilled and thus no consideration for enforcing such an agreement. Justice Field dissented from this decision on the basis that partial performance should constitute sufficient consideration to enforce a contract in certain circumstances. He argued that if one party has already substantially complied with their contractual obligations then they should have recourse against another party who fails to fulfill theirs; otherwise, contracts would become unenforceable when only partly completed due to lack of consideration or mutuality of obligation - something which he felt would lead to injustice and uncertainty in commercial transactions.