Mo. R. Civ. P. 55.31
As amended through May 1, 2024
A party cannot object that no demand for the subject matter of a civil action was made prior to its institution unless it is expressly set up by way of defense in the answer or reply and is also accompanied with a tender of the amount or thing that is due; in which case, if the plaintiff will further prosecute the civil action and shall not recover a greater sum or more than is tendered, the plaintiff shall pay all costs. This provision is applicable as well to actions for property as for money; when property is tendered the damages for its detention, if any, shall also be tendered.
Mo. R. Civ. P. 55.31