Relating to: adopting changes to the federal Internal Revenue Code for state tax purposes and modifying administrative rules related to such changes. (FE)

AB 870 updates Wisconsin's state tax code to align with federal tax law changes by adjusting reference dates for the Internal Revenue Code. For taxable years starting after December 31, 2025, Wisconsin will use the federal tax code as amended through December 31, 2025 - instead of the previous 2022 cutoff - while excluding specific federal tax provisions enacted after 2025. The bill directly affects Wisconsin taxpayers and fiduciaries (excluding nuclear trust funds) by modifying how state tax calculations account for federal law differences, including exclusions for certain deductions and credits. It creates new administrative rules to handle discrepancies between state and federal tax treatments, such as differences in depreciation methods or asset basis.