§ 426.50.Amendment of Pleading Or Filing Cross-Complaint to Assert Cause of Action Subject to Requirements of Article

Section 426.50 is the safety valve for § 426.30's otherwise harsh use-it-or-lose-it rule. A party who failed to plead a related cause of action — whether through oversight, inadvertence, mistake, neglect, or some other cause — may apply to the court for leave to amend the pleading, or to file a cross-complaint, asserting that cause of action at any point during the action.

The court's role is largely non-discretionary once good faith is shown: after giving notice to the adverse party, the court must grant leave, on terms that are just to the parties, if the party who failed to plead the cause acted in good faith. The statute closes with an instruction that the subdivision be liberally construed to avoid forfeiture of causes of action.

That liberal-construction directive reflects a deliberate policy choice: the compulsory cross-complaint rule exists to consolidate related claims, not to punish good-faith oversights with permanent forfeiture. A party facing a lost claim under § 426.30 should look here first, before assuming the claim is gone for good.