Supplemental Jurisdiction Explained: 28 U.S.C. § 1367 (1L Civ Pro)

Every claim in federal court needs its own basis for subject-matter jurisdiction — usually a federal question under 28 U.S.C. § 1331 or diversity under § 1332. Supplemental jurisdiction is the doctrine that lets a federal court hear an additional claim that has no independent basis of its own, because it is closely tied to a claim that does.

Without it, a plaintiff with one federal claim and one closely related state claim would have to split the dispute across two courthouses. Supplemental jurisdiction, codified at 28 U.S.C. § 1367, keeps related claims together in one case. The trick on exams is that § 1367 has three subsections that do three different jobs — grant, take away, and give discretion.

The modern test traces to United Mine Workers v. Gibbs (1966), which allowed "pendent" jurisdiction over a state claim that shared a "common nucleus of operative fact" with a federal claim — facts so intertwined that a plaintiff would ordinarily expect to try them in one proceeding.

After the Supreme Court narrowed the doctrine in Finley v. United States (1989), Congress responded by enacting § 1367 in 1990, codifying and clarifying when supplemental jurisdiction is available. So when you analyze a problem, you apply the statute first — but the Gibbs "common nucleus" idea lives on inside § 1367(a).

Section 1367(a) is the grant of power. It says that once a district court has original jurisdiction over a claim, it also has supplemental jurisdiction over all other claims that are "so related" to that claim that they "form part of the same case or controversy under Article III."

In practice, "same case or controversy" is read as the Gibbs common-nucleus-of-operative-fact test: do the claims arise from the same underlying events? Section 1367(a) also expressly extends supplemental jurisdiction to claims that bring in additional parties, which is what made it broader than the old pendent-claim doctrine.

So the first step is always: is there at least one anchor claim with its own jurisdiction, and does the extra claim share a common nucleus of fact with it? If yes, § 1367(a) grants power — unless § 1367(b) takes it back.

Section 1367(b) is the one students miss. It applies only when the case is in federal court solely because of diversity jurisdiction. In that situation, it withdraws supplemental jurisdiction over certain claims that would undermine the complete-diversity rule.

Specifically, in a diversity-only case, there is no supplemental jurisdiction over claims by plaintiffs against persons made parties under Rule 14 (impleader), Rule 19 (required joinder), Rule 20 (permissive joinder), or Rule 24 (intervention), nor over claims by plaintiffs proposed to be joined under Rule 19 or seeking to intervene under Rule 24, when allowing the claim would be inconsistent with the requirements of § 1332.

The point is to stop plaintiffs from using supplemental jurisdiction as a back door to sue a non-diverse defendant they could not have sued directly. Note the asymmetry: the bar targets claims by plaintiffs. Defendants' claims — like a defendant impleading a non-diverse third party under Rule 14 — are generally not barred by § 1367(b).

Even when subsections (a) and (b) permit supplemental jurisdiction, § 1367(c) gives the district court discretion to decline it. A court may decline supplemental jurisdiction if: (1) the claim raises a novel or complex issue of state law; (2) the state claim substantially predominates over the claims with original jurisdiction; (3) the court has dismissed all claims over which it had original jurisdiction; or (4) in exceptional circumstances, there are other compelling reasons to decline.

Number (3) is the workhorse. If the federal anchor claim is dismissed early, courts routinely decline supplemental jurisdiction over the remaining state claims and send them back to state court, consistent with the Gibbs instinct that federal courts should avoid needless decisions of state law.

Exxon Mobil Corp. v. Allapattah Services (2005) resolved how § 1367 interacts with the diversity requirements. The Court held that in a diversity case where at least one plaintiff satisfies the amount-in-controversy requirement, § 1367 can supply supplemental jurisdiction over the claims of additional plaintiffs who fall short of that amount — for example, additional class members.

But — and this is the line to remember — Allapattah did not loosen the complete-diversity rule. Supplemental jurisdiction can cure a shortfall in the amount in controversy; it cannot cure a lack of complete diversity. If a co-plaintiff is non-diverse, § 1367(b) still bars the claim.

The classic exam trap is a diversity-only case where a plaintiff tries to add a claim against a non-diverse defendant joined under Rule 20, then invokes supplemental jurisdiction to keep it. Section 1367(b) exists precisely to block that move — it would be inconsistent with § 1332's complete-diversity requirement.

Work the subsections in order: (a) is there a common nucleus? (b) is this a diversity-only case with a barred plaintiff-side claim? (c) should the court decline anyway? Skipping straight to "they're related, so it's fine" is how points are lost.