The HSA last-month rule and its 13-month testing-period trap

Becoming eligible for a Health Savings Account (HSA) halfway through the year feels like arriving late to a party where the open bar is still pouring. A quirk in the tax code, called the last-month rule, lets you fill your glass as if you had been there since January. The short answer. If you are an HSA-eligible individual on the first day of the last month of your tax year — December 1 for anyone who files on a calendar year — you may contribute the full year’s limit even though you were eligible for only part of the year. The catch is that this generosity comes with a tripwire: a testing period that stretches roughly thirteen months into the future, and if you fail it, the Internal Revenue Service (IRS) takes back the favor with interest.

This is the deep dive into Part III of Form 8889, where the last-month rule and its testing period actually live. If you want a walkthrough of the whole form, line by line, that lives in our Form 8889 line-by-line guide; here we stay narrowly on the one calculation that catches people off guard.

How the last-month rule actually works

Ordinarily, your HSA contribution limit is prorated. Eligible for six months of the year, you may contribute roughly half the annual figure. The last-month rule, described in IRS Publication 969 and reported in Part III of Form 8889, overrides that proration. If you hold HSA eligibility on the first day of December, the IRS treats you as if you had been eligible for all twelve months, so you may contribute the full annual amount.

Consider a self-only filer in 2025 who switches to a high-deductible health plan in October and is eligible only for October, November, and December — three months. Prorated, that person could contribute roughly a quarter of the $4,300 self-only limit for tax year 2025. Under the last-month rule, the same person may contribute the entire $4,300. A family filer in that position could move up to the $8,550 family limit, and anyone who is 55 or older may add the $1,000 catch-up contribution on top. The appeal is obvious: a much larger tax-deductible contribution from a year in which you were eligible for only a sliver of the calendar.

The catch: a testing period that runs into next year

Here is the part that the brokerage marketing emails tend to leave out. Using the last-month rule obligates you to remain an HSA-eligible individual throughout a testing period. That period begins with the last month of the tax year and ends on the last day of the twelfth month following it. For a 2025 contribution, that means the window runs from December 1, 2025 through December 31, 2026 — about thirteen months in plain reading, well into the following tax year.

In other words, the decision you make for one tax year reaches forward and binds the next. You cannot simply pocket the full contribution in 2025 and then change your health coverage freely in 2026. The IRS is essentially extending you credit on the assumption that you will keep qualifying.

What failing the test costs you

If you fail the testing period — meaning you lose HSA eligibility at any point during it for any reason other than death or disability — the penalty is twofold, and it is steeper than most people expect.

First, you must include in income the amount you contributed that you could not have contributed but for the last-month rule. That is the difference between what you actually put in and what you would have been allowed to contribute on a prorated basis. Second, that same amount is subject to an additional 10% tax. So you both pay ordinary income tax on the clawed-back portion and hand over an extra penalty on top. The income inclusion and the 10% surcharge land in the year you fail the test, not the year you made the contribution.

Return to our October-eligible self-only filer. Suppose that person used the last-month rule to contribute the full $4,300 for 2025, then in mid-2026 enrolled in Medicare, switched back to a traditional non-high-deductible plan, or otherwise lost eligibility before December 31, 2026. The portion attributable solely to the last-month rule — the slice above what three months of proration would have allowed — gets added back to 2026 income and is hit with the 10% additional tax. The full-funding head start turns into a tax bill plus a penalty.

The triggers are ordinary life events, which is exactly why this trap is so easy to spring. Enrolling in Medicare at 65, taking a new job with a plan that is not high-deductible, getting married onto a spouse’s non-qualifying coverage, or simply choosing a different plan during open enrollment can all end your eligibility mid-testing-period. None of them feel like tax decisions when you make them.

The opt-out almost nobody mentions

The reassuring news is that the last-month rule is entirely optional. You are never required to use it. If you would rather not carry thirteen months of eligibility risk, you can contribute only the prorated amount for the months you were actually eligible. That smaller contribution carries no testing-period exposure at all, because the testing period only attaches to the extra dollars the last-month rule made possible.

This is the quiet judgment call. If you are confident your eligibility will hold steady — you intend to keep the same high-deductible plan, you are years away from Medicare, and your life is stable — the last-month rule lets you front-load a meaningful tax deduction and start compounding sooner, which matters a great deal if you treat the account as a long-term vehicle rather than a spending pot. We make that case at length in our piece on using an HSA as a stealth retirement account. If your coverage is in flux, prorating is the conservative path that keeps the IRS out of your next return.

A worked comparison helps. The October-eligible filer who prorates contributes roughly a quarter of $4,300 and walks away with no future obligation. The same filer who uses the last-month rule contributes the full $4,300, secures a larger deduction now, but accepts that an eligibility lapse before December 31, 2026 unwinds part of it with the 10% tax attached. Neither choice is wrong; they price risk differently.

One more distinction worth keeping straight: the testing period attached to the last-month rule is not the same animal as the eligibility rules that separate accounts in the first place. If you are still untangling which tax-advantaged health account you even have, our comparison of an HSA versus an FSA sorts out the basics before any of this Part III math becomes relevant. Once you know you are funding a genuine HSA, the last-month rule is the lever — and the testing period is the fine print you read before you pull it.