I had a HDHP every month in 2024, and every month except Dec in 2025. Why is Turbotax saying "we calculated $xx income to include under the last-month rule"?

[Mike9241]

this is a TurboTax bug. It thinks you used the last-month rule (LMR)  in 2024 to make a full-year's contribution. If used, the rule requires you to maintain HDHP coverage for every month in 2025. You did not use the LMR. The workaround is to go back to the 2024 data and say you had no coverage for any month. 

[Wildcard]

This is a bug that has not been fixed as of Feb. 23

The bug is with the January 2024 HDHP coverage selection.  If you select that you had HDHP coverage in January 2024, TurboTax software does the opposite and thinks that you did not have coverage and applies the extra income penalty.

If you select "none" in January 2024 it thinks you did have coverage and does not apply the income penalty.

I will not pay $99 and have to lie to the software to get it to calculate my taxes correctly.  I'm switching to another company after this.