Wis. Stat. § 807.05: Stipulations.

No agreement, stipulation, or consent between the parties or their attorneys, in respect to the proceedings in an action or special proceeding shall be binding unless made in court or during a proceeding conducted under s. 807.13 or 967.08 and entered in the minutes or recorded by the reporter, or made in writing and subscribed by the party to be bound thereby or the party’s attorney.