Medicare Advantage vs. Original Medicare (2026) -- and the HSA trap if you're still working past 65

Medicare's annual Open Enrollment runs October 15 to December 7, 2026. The choice underneath it -- Original Medicare or Medicare Advantage -- determines which doctors you can see, whether you can buy a Medigap supplement, and whether there's any cap on what a bad year could cost you. If you're still working past 65 and covered by an employer high-deductible health plan (HDHP), there's a second, less obvious decision stacked on top of it: enrolling in Medicare at the wrong time can quietly turn HSA contributions you've already made into a tax problem. Every figure below is checked against CMS's official 2026 announcement and Medicare.gov; the HSA mechanics are checked against Medicare.gov's own enrollment rules and IRS Publication 969.

Original Medicare vs. Medicare Advantage

Original Medicare is Part A (hospital) and Part B (medical) run directly by the federal government. Medicare Advantage (Part C) is a private insurance plan, approved by Medicare, that replaces Original Medicare -- Medicare.gov describes these as "bundled" plans that "include Part A, Part B, and usually Part D." The two options differ in four ways that matter for planning, not just price:

The out-of-pocket maximum is the one people most often get backwards. Medicare.gov is explicit that Original Medicare has "no yearly limit on what you pay out-of-pocket, unless you have supplemental coverage" -- a bad year of hospital stays and treatment on Part A and B alone has no ceiling. Medicare Advantage plans are required to have a yearly limit; CMS's 2026 rules cap what any plan's in-network limit can be at $9,250 (down from $9,350 in 2025), with a combined in-and-out-of-network ceiling of $13,900. That's the maximum allowed, not what most people actually pay: plans are free to set their limit lower, and typical in-network limits run well under the cap. Either way, once you hit your plan's limit, Medicare Advantage covers 100% of covered services for the rest of the year -- a backstop Original Medicare alone doesn't have.

The 2026 cost baseline

These are from CMS's official 2026 Medicare Parts A & B premiums and deductibles announcement (November 14, 2025) and CMS's Medicare Advantage premium projection (September 26, 2025):

Part A is premium-free for almost everyone because it's funded by payroll taxes paid over a working career -- CMS puts the premium-free share at "approximately 99% of Medicare beneficiaries," the threshold being 40 quarters (10 years) of Medicare-covered employment. Part B always carries a premium; higher earners pay more under IRMAA, covered in the IRMAA guide. The $14.37 average Medicare Advantage premium is additional to Part B, not instead of it -- you keep paying Part B either way, because Medicare Advantage plans are still required to cover everything Original Medicare does.

Still working past 65? The HSA/HDHP trap

This is the part most comparisons skip. If you're covered by a qualifying HDHP through a current employer (not retiree coverage, not COBRA) and you delay enrolling in Medicare because you're still working, you can keep contributing to your HSA. The rule, straight from IRS Publication 969: "Beginning with the first month you are enrolled in Medicare, your contribution limit is zero." That's any part of Medicare -- including Part A alone, which is premium-free and which Social Security enrolls most people in automatically once they claim retirement benefits.

The trap is timing. Medicare.gov's own enrollment rules for signing up for premium-free Part A after your initial enrollment window say: "Your Part A coverage starts 6 months back from when you sign up or when you apply for benefits from Social Security... Coverage can't start earlier than the month you turned 65." So if you keep working, keep contributing to your HSA, and then apply for Medicare (or for Social Security, which auto-enrolls you in Part A) three years after turning 65, your Part A entitlement gets backdated six full months -- not three years, just six months, and never before your 65th birthday month. Publication 969 is direct about what that does to contributions made in that window: "This rule applies to periods of retroactive Medicare coverage," meaning any HSA contributions made during those backdated months become excess contributions, because you weren't actually HSA-eligible for them after all.

Excess HSA contributions carry a 6% excise tax (IRS Form 5329) for every year they stay in the account. Publication 969's fix is a deadline, not a penalty you're stuck with: "You withdraw the excess contributions by the due date, including extensions, of your tax return for the year the contributions were made," and any earnings on the withdrawn amount get reported as income in the year you take it out. Miss that deadline and the 6% applies for every year the excess sits there.

Worked example

Contributing $500 a month to an HSA while on an employer HDHP, and applying for Medicare Part A 14 months after turning 65 (well past the initial enrollment window): Part A backdates the full 6 months, the maximum allowed. The last 6 months of $500 contributions, $3,000, were made during months you were no longer actually eligible -- an excess contribution of $3,000, and a $180 excise tax (6% of $3,000) for every year it isn't withdrawn. Apply right at 65 instead, and there's no backdating at all, because Part A simply starts the month you turn 65 -- no retroactive window, no excess.

The practical fix is the one Medicare.gov's own rule points to: stop HSA contributions at least 6 months before you apply for Medicare Part A or for Social Security after 65, whichever comes first, if you're still on an employer HDHP. Run your own numbers -- current age, how many months after 65 you plan to enroll, and your monthly contribution -- in the HSA/Medicare excess contribution calculator, which flags the likely excess and the 6% cost if it isn't withdrawn in time.

See also

For the rest of the partial-year HSA math -- starting an HDHP mid-year, switching between self-only and family coverage, the last-month rule and its testing period -- see the partial-year HSA guide and the HSA contribution limit calculator it's built on. If your Medicare decision is also an income-timing decision -- a Roth conversion or a QCD choice this December -- the IRMAA guide covers the 2-year lookback that sets your Part B and Part D premiums.

Every premium, deductible and coinsurance figure above is from CMS's official 2026 Medicare Parts A & B premiums and deductibles announcement (November 14, 2025) and CMS's 2026 Medicare Advantage premium projection (September 26, 2025); the out-of-pocket maximum is the 2026 CMS-mandated ceiling for Medicare Advantage plans. Coverage and enrollment rules are from Medicare.gov; the HSA rules are from IRS Publication 969. This explains the mechanics, not personalized advice -- your own plan's actual out-of-pocket limit, network, and premium will differ from the averages and caps shown here, and state Medicaid programs and IRMAA can change what you actually pay.