This question was answered in Mahnami, a recent case coming from the Court of Appeals of Idaho. Mahnami v. Mahnami, 156 Idaho 338, 2014 WL 1884896 (Idaho App. 2014). In this case, a divorced couple asked the magistrate judge to resolve four disputes regarding their minor child. These disputes were not specific to custody of the minor child. Rather, the disputes involved the child's involvement in soccer, the child's need for additional assistance with her eyesight, counseling for the child's emotional needs, and production of insurance claims between the parties. It was clear from the pleadings that neither party anticipated changing the custody of the child as a resolution to the disputes in the case. Significantly, neither party filed a petition to modify the custody arrangements. To solve the dispute, however, the magistrate judge modified custody of the minor child by determining that the mother would have the ultimate authority to make a decision regarding legal custody of the child in the event that the couple cannot agree. The Idaho Court of Appeals determined that the magistrate court acted improperly because the court did not modify the custody of the parties in accordance with Idaho law.
The law in Idaho states that in order to modify a custody agreement, the petitioner must prove that there has been a substantial and permanent change. The petitioner must then file a petition that specifically sets forth the change in circumstances and the modification sought. Without filing this petition, the judge typically lacks the authority to modify a custody agreement. However, the Court of Appeals noted in this case that according to Rule of Civil Procedure 15(b), "if issues that are not included in the pleadings are tried by express or implied consent, these issues are treated as if they had been included in the pleadings." However, for issues to be resolved that did not appear in the pleadings, the parties must have sufficient notice that the issue would be tried, they must have the opportunity to address these issues with evidence, and both parties need to understand that the evidence was being presented in relation to the unpled issue.
In this case, the parties did not file petitions to modify the custody agreement, they did not consent to trying the issue of custody, and they did not have the opportunity to present evidence to specifically address the custody matter. Therefore, the magistrate judge in this case exceeded his authority to change the legal custody of the parties.