California Codes, Code of Civil Procedure - CCP, Part 2 — Of Civil Actions, Title 6 — Of the Pleadings in Civil Actions, Chapter 2 — Pleadings Demanding Relief, Article 2 — Compulsory Cross-Complaints, Section 426.50

A party who fails to plead a cause of action subject to the requirements of this article, whether through oversight, inadvertence, mistake, neglect, or other cause, may apply to the court for leave to amend his pleading, or to file a cross-complaint, to assert such cause at any time during the course of the action. The court, after notice to the adverse party, shall grant, upon such terms as may be just to the parties, leave to amend the pleading, or to file the cross-complaint, to assert such cause if the party who failed to plead the cause acted in good faith. This subdivision shall be liberally construed to avoid forfeiture of causes of action.