Cliff-riding: why optimizing to $1 under a threshold is fragile

Last reviewed July 2026 · 5 min read

Most of the tax code slopes: earn a dollar more, pay some cents more. But a few of the rules that matter most in retirement are cliffs — one dollar of extra income and a benefit vanishes or a surcharge lands in full. Optimizers love cliffs: the “perfect” plan earns exactly $1 less than the threshold. This article is about why that perfect plan is fragile, and why a deliberate safety margin is usually worth more than the income it gives up.

The two big cliffs in a retiree’s path

The ACA subsidy cliff. With the enhanced credits expired after 2025, marketplace premium subsidies for 2026 end entirely when MAGI exceeds 400% of the federal poverty level. For a two-person household in the 48 contiguous states that line sits at $84,600 of MAGI (4 × $21,150) for 2026 coverage. Just under it, your premium contribution is capped at 9.96% of income; one dollar over, the entire subsidy is repaid — for a couple in their early 60s, whose age-rated benchmark premium is near the maximum, that single dollar can cost well over $10,000.

The IRMAA tiers. Medicare’s income surcharge is a staircase of cliffs. For 2026, MAGI above $218,000 on a joint return (above $109,000 single) adds $81.20 per month to each person’s Part B premium and $14.50 to Part D — about $2,297 a year for a couple both on Medicare, for crossing by $1. Four more tiers wait above it. And because IRMAA looks at MAGI from two years earlier, the dollar that crosses the line does its damage on a delay.

Why $1 under the line is a fantasy

Riding the cliff edge assumes you control MAGI to the dollar. You don’t. In December, mutual funds distribute capital gains you didn’t ask for and can’t precisely predict. Interest and dividend income wobble with rates and payout dates. A CD matures, a bond fund pays more than last year, a corrected 1099 arrives in March. Any one of these can add a few hundred dollars of MAGI — and a plan calibrated to $1 under the line loses the entire subsidy or eats the full surcharge over a rounding error.

There is a second, subtler problem: the thresholds themselves move. ACA percentages and poverty guidelines are re-indexed every year, and IRMAA tiers are announced only weeks before the year begins — yet the two-year lookback means a Roth conversion today is measured against IRMAA thresholds that have not been published yet. A plan built years ahead is aiming at a line it cannot see, using income it cannot fully control.

Sizing a margin: the asymmetry argument

The case for a buffer is that the costs are lopsided. Leaving, say, $5,000 of Roth conversion on the table costs you the modest benefit of converting those dollars at today’s bracket — typically hundreds of dollars of eventual tax difference. Overshooting a cliff by the same accident costs thousands, immediately. When the downside is 5–20× the upside, the break-even probability of a MAGI surprise is tiny — and MAGI surprises are not tiny. A common-sense margin is a few thousand dollars below the nearest cliff, wider when your income has more moving parts (funds in taxable accounts, variable interest, side income).

Cliffs compound in a simulation

In a Monte Carlo view this gets sharper: each simulated future has its own inflation path, so the indexed thresholds sit at slightly different heights in every path. A strategy that clears the cliff in the median future may cross it in a third of the others. The question is not “does my plan clear the cliff?” but “how close does it come, in how many futures?”

Try it in Deorbit Plan

The Strategy panel has a cliff-margin guard — “Stay this far below the nearest cliff” — that caps fill-bracket Roth conversions so MAGI keeps your chosen buffer under the nearest ACA or IRMAA boundary. Every result in the Strategy Lab reports the closest cliff approach across the plan (“$2.1K under IRMAA tier 2 in 2054”), flagging in warning color when the gap falls under $5,000. Run the conversion sweep with different margins and watch how much — or how little — lifetime spending you actually give up for the safety.

Educational content only — not financial, tax, or investment advice.

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