Idaho Code § 15-2-701

A contract to make a will or devise, or not to revoke a will or devise, or to die intestate, if executed after the effective date of this act, can be established only by (1) provisions of a will stating material provisions of the contract; (2) an express reference in a will to a contract and extrinsic evidence proving the terms of the contract; or (3) a writing signed by the decedent evidencing the contract. The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.