Legal Question in Business Law in California

A judge will determine the breach of contract.

A judge determines the breach of contract on a motion or a demurrer, if the issue is properly raised. If there is a dispute over facts that may or may not give rise to a breach, then that goes to a jury if properly requested by either party.

While the question would be "finally" resolved in court, by a judge or jury, and perhaps by a panel of judges in an appellate court, or by an arbitrator or arbitrators if the contract called for arbitration, one or both parties can get a quick, inexpensive and pretty accurate forecast of how a court would rule. Simply take the contract to an attorney experienced in contract law after arranging with him or her for a free initial consultation. I'd be willing to do so.

You said "years ago". The statute of limitations period within which time you must file a breach of contract claim is 4 years from the date of the claimed breach. You may have an absolute defense. See a lawyer.