HSA Contributions: HDHP Individual in 2025 changing to HDHP Family in 2026. Need help understanding “Last-Month Rule” and “Testing Period”

I maxed out my HSA contribution for my individual plan ($4300) in 2025. During open enrollment, I added my domestic partner to my medical insurance and will have the family plan starting January 1, 2026. I checked my HSA account and it’s listing both a “Family limit” and “Individual limit”, where $4250 is still left for the 2025 “Family limit.”

Since the “Family limit”is showing up on my HSA account and seems to be including the contributions I made to my “Individual limit”, am I able to max out the “Family limit” for 2025 even though I only had insurance for myself? Can my domestic partner contribute the family limit into their HSA for 2025 too or would they only be eligible to contribute the family limit for 2026 if their benefits don’t go into effect until January 1, 2026? At least moving forward from 2026, my understanding is that we can both contribute the family limit to our respective HSA accounts.

I am just now hearing and learning about the “Last-Month Rule” and “Testing Period.” I want to max out my savings as much as possible without getting penalized for over-contributing. We will have the family HDHP for all of 2026.

Appreciate any clarification and many thanks in advance!