HSA last month rule testing period, if I am on HDHP family on Dec. 1, 2024, can I maintain HDHP individual from Jan 1 to Dec. 31, 2025? or it has to HDHP family plan?

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You only have to be an "eligible individual", meaning you are covered by an HDHP and have no other disqualifying coverage or situations. If contribute to the family max using the last month rule in 2024, and later change to single coverage during the 2025 testing period, that does not create a situation where part of your 2024 contributions are ineligible or subject to penalty.