Private immigration-detention contractor GEO asserted that the Yearsley government-contractor doctrine shielded it from detainees’ claims and sought an immediate appeal after the district court rejected the defense. The Supreme Court held that the denial was not immediately appealable under the collateral-order doctrine. Yearsley supplies a defense on the merits when a contractor acted within validly conferred federal authority; it does not create an immunity from suit comparable to sovereign or qualified immunity. Because the asserted right can be effectively reviewed after final judgment, the court of appeals properly dismissed the interlocutory appeal. The judgment was affirmed.