Substitute Senate Bill No. 1558, also known as Public Act No. 25-172, mandates the Attorney General to study and propose measures to protect Connecticut residents from income taxes imposed by other states or the District of Columbia on income earned while the resident was physically present in Connecticut. The Attorney General is required to submit a report with findings and recommendations to the General Assembly's finance committee by January 1, 2026.

Additionally, the bill introduces a new tax credit for Connecticut residents who meet specific criteria related to taxes paid to other jurisdictions. This credit allows eligible residents to receive a refund equal to sixty percent of the taxes owed to Connecticut due to adjustments in credits for taxes paid to other states. To qualify, residents must have paid income tax to another jurisdiction, been denied a refund, filed an appeal, and received a final decision resulting in a refund. The bill also stipulates that no penalties or interest will be imposed on late tax payments if they are due to a reduction in the credit resulting from a refund from another state, provided certain conditions are met.