None, the IRS does not care about your court order unless it is dated before 2009 or about your custody schedule. The IRS goes by its own rules and will award the dependency deduction to the custodial parent if both parents try to claim the child. Your remedy against your ex is to take him/her back to court.
To claim the dependent, your only option will be to paper file (mail) your return. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year or more) thereafter, you will receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you does not file an amended return, unclaim the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner. The IRS will apply its tie-breaker rules.
Under the tie-breaker rule, the child is treated as a qualifying child:
- The parent, if only one of the persons is the child's parent,
- The parent with whom the child lived the longest during the tax year, if two of the persons are the child's parents and they do not file a joint return together,
- The parent with the highest AGI if the child lived with each parent for the same amount of time during the tax year, and they do not file a joint return together,
- A non-parent, if no parent claims the child as a qualifying child although he or she may do so and only if the non-parent's AGI is higher than the highest AGI of any parent who may claim the child, or
- The person with the highest AGI, if none of the persons is the child's parent.
The winner gets the tax benefits; the loser gets to pay the IRS back with penalties and interest. The custodial parent almost always wins. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332) or if it's spelled out in a pre-2009 divorce decree.
https://www.thebalance.com/claiming-same-dependent-audit-risk-3193030
IRS Tie Breaker Rules
