K.C.

The common-law doctrine of ancillary jurisdiction over related claims, codified as part of a federal court’s supplemental jurisdiction under 28 U.S.C. § 1367 , is generally well understood. Yet, as the Supreme Court observed, the “doctrine of ancillary jurisdiction can hardly be criticized for being overly rigid or precise,” because the more obscure doctrine of ancillary jurisdiction over collateral proceedings remains a matter of case law. Kokkonen v. Guardian Life Ins. Co. of Am., 511 U.S. 375, 379 , 114 S.Ct. 1673 , 128 L.Ed.2d 391 (1994); see 13 Charles Alan Wright et al., Federal Practice & Procedure § 3523 (3d ed.). The present appeal involves the latter — ancillary jurisdiction over ‘proceedings related to, but technically separate from, a federal lawsuit. Under this doctrine, a federal court may exercise ancillary jurisdiction over collateral proceedings in two distinct contexts that are relevant here: to enforce a settlement agreement, and to resolve an attorney’s fees dispute.

Four juvenile plaintiffs, by and through their respective guardians, and the American Diabetes Association (collectively, “Plaintiffs”) filed a putative class action against Jack…