What impact does "complete diversity" have on supplemental jurisdiction?

The concept of "complete diversity" refers to the requirement under federal diversity jurisdiction that all plaintiffs be citizens of different states from all defendants. When considering supplemental jurisdiction, which allows federal courts to hear additional claims that are closely related to those within their original jurisdiction, complete diversity can indeed have a significant impact, particularly when it comes to state law claims.

When a federal court has original jurisdiction over a claim based on diversity, it must ensure that any additional claims added under supplemental jurisdiction do not destroy the complete diversity required for the primary claim. For instance, if a plaintiff wishes to assert a state law claim against a defendant who is a citizen of the same state as the plaintiff, it would undermine the complete diversity and could compel the court to dismiss that claim. Therefore, while supplemental jurisdiction enables bringing additional claims, it is limited by the need to maintain complete diversity when state law claims are involved. This ensures that federal courts uphold the jurisdictional requirements established by statute.

Overall, the limitation imposed by complete diversity means that not all claims can be brought together in a single action if those claims would compromise the diversity jurisdiction necessary for federal jurisdiction.