What AGI amount do I use for the prior year AGI if only one spouse on the joint return earned all the wages?

Both you and your spouse on the joint return will use the total AGI amount from the originally filed return. Do not divide the AGI between you and your spouse. For example, if your AGI on your 2024 tax return is $20,523 then enter 20,523 for the prior year AGI for yourself and 20,523 for the prior year AGI for your spouse.