Whether a post-filing change in a continuing party's citizenship can cure the absence of diversity jurisdiction at the time the action was filed.
Holding
No. A post-filing change in the citizenship of a continuing party cannot cure a diversity-jurisdiction defect that existed when the complaint was filed.
Reasoning
The Court applied the longstanding time-of-filing rule: jurisdiction in a diversity case depends on the facts existing when the action is commenced. The rule applies whenever subject-matter jurisdiction is challenged, including after trial or for the first time on appeal. Since Atlas was a Mexican citizen at filing through its Mexican partners, and Dataflux was Mexican, the parties were not completely diverse when this action began.
Atlas's later reorganization changed the partnership's citizenship but did not change the party to the lawsuit. Atlas remained the sole plaintiff throughout. Under Carden v. Arkoma Associates, a partnership is treated as a single artificial entity whose citizenship is determined by the citizenship of all its partners; looking to partners identifies the entity's citizenship, not additional litigants who may be treated as separately dropped from the case.
The Court distinguished the accepted cure in cases where a dispensable nondiverse party is dismissed under Federal Rule of Civil Procedure 21. Dismissing such a party changes the actual lineup of litigants and leaves a completely diverse set of parties. Here, by contrast, no party was dismissed; the continuing plaintiff simply changed its internal composition and, with it, its citizenship.
The Court declined to create a new exception based on finality or judicial economy. Allowing a post-filing citizenship change to validate a suit would depart from nearly two centuries of precedent and create uncertainty about when jurisdictional defects may be cured. Clear, stable jurisdictional rules reduce collateral litigation over federal-court authority.