Mo. Rev. Stat. § 474.155: Contract to make will or devise, revoke or not revoke will or devise, or to die intestate, how established.

A contract to make a will or devise, to revoke or not to revoke a will or devise, or to die intestate, if executed after January 1, 1981, 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.