Res judicata prevents a party from bringing the same claim twice. Once a court has entered a final judgment on the merits in a dispute between two parties, res judicata bars either party from filing a new lawsuit asserting the same claim -- including theories and arguments that were never actually raised in the first case but could have been.

This 'could have been raised' feature is what distinguishes res judicata from collateral estoppel (issue preclusion), which bars relitigation only of issues actually litigated and decided, and can apply even in a later suit on a different claim. Res judicata operates at the level of the entire claim or cause of action; collateral estoppel operates at the level of individual issues within it.

Courts generally require: (1) a final judgment on the merits in the prior action; (2) rendered by a court of competent jurisdiction; (3) involving the same parties, or parties in privity with them; and (4) the same claim or cause of action as the one now being asserted. A dismissal with prejudice, a judgment after trial, and many summary judgments qualify as final judgments on the merits; a dismissal for lack of jurisdiction or without prejudice typically does not.

Privity extends the bar beyond the literal named parties to those whose interests were adequately represented in the first action -- successors in interest, certain classes of indemnitors and indemnitees, and parties in a sufficiently close legal relationship with an original litigant.

Most jurisdictions, following the Restatement (Second) of Judgments, define 'same claim' broadly using a transactional test: claims arising out of the same transaction or series of connected transactions are treated as a single claim for preclusion purposes, regardless of how many distinct legal theories could describe the underlying conduct. A plaintiff who sues on a breach-of-contract theory and loses generally cannot later sue the same defendant over the same transaction on a fraud or negligence theory, even if fraud was never pleaded the first time.

This is why res judicata is often described as barring not just claims actually litigated, but every claim that could have been litigated arising from the same underlying facts -- it forces plaintiffs to bring all related theories in one action or lose them.

Res judicata (claim preclusion) bars an entire second lawsuit on the same claim between the same parties, regardless of which issues were actually litigated. Collateral estoppel (issue preclusion) is narrower and can arise even in a lawsuit on a completely different claim: it bars relitigating a specific factual or legal issue that was actually litigated, actually decided, and necessary to the judgment in the prior case.

The two doctrines are frequently pleaded together but require separate analysis -- a party can lose a res judicata argument (because the second suit involves a genuinely different claim, not the same transaction) while still winning a collateral estoppel argument on a specific issue common to both suits.

In Juricratic, res judicata is modeled as a case-dispositive gate distinct from the collateral-estoppel issue-level gate already in the system: it operates on the entire claim, activated once a prior final judgment between the same parties (or privies) covering the same transaction is entered into the record. Because the transactional test sweeps in theories never actually pleaded the first time, a simulation should flag every legal theory arising from the same underlying facts as an at-risk claim path the moment a related prior judgment appears in the matter's procedural history.

This makes res judicata one of the highest-value early dials to check in any matter with related prior litigation -- it can eliminate entire claim paths from the simulation before the merits are ever war-gamed.