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Taxation: ISOs, NSOs, and the AMT

The tax treatment of options is dictated by their classification as either Incentive Stock Options (ISOs) or Non-Qualified Stock Options (NSOs).

Non-Qualified Stock Options (NSOs). NSOs carry no tax advantages. There is no tax at the time of grant. Upon exercise, the spread (FMV minus strike price) is taxed as ordinary compensation income. This spread is subject to FICA taxes and federal/state income tax withholding. The employer reports this as wages on Form W-2. The exercise establishes a new tax basis equal to the exercise-date FMV. Any subsequent appreciation is taxed as capital gains when the shares are sold.

Incentive Stock Options (ISOs). ISOs qualify for preferential tax treatment under IRC §422 if specific rules are met:

If these holding periods are satisfied, no regular income tax is due at exercise. When the shares are sold, the entire gain (sale price minus strike price) is taxed as a long-term capital gain.

If you sell the shares before meeting both holding periods, it is a disqualifying disposition. The spread at exercise is taxed as ordinary income in the year of sale, and any additional gain is taxed as capital gains.

Alternative Minimum Tax (AMT) Trap. While ISO exercises are exempt from regular income tax, the spread at exercise is an adjustment item for the Alternative Minimum Tax (AMT) under IRC §56(b)(3). If the exercise triggers AMT, you must pay tax on this paper gain. This creates a severe financial risk: if the stock price declines after exercise and you cannot sell due to lockups or illiquidity, you remain liable for AMT calculated on the exercise-date FMV, potentially paying taxes that exceed the final liquidation value of the shares. This is the canonical way startup employees go broke on a winning lottery ticket — exercising into a private, illiquid stock and owing real cash on a paper gain that never materializes.

Disqualifying dispositions and the AMT escape hatch. The ordinary income on a disqualifying disposition is the lesser of the spread at exercise or the actual gain at sale, so a stock that fell after exercise limits the ordinary-income hit. This is also the one clean way to neutralize the AMT trap: if you sell the shares in the same calendar year you exercised, the exercise stops being an AMT adjustment at all — it collapses into an ordinary-income event like an NSO. You forfeit the preferential long-term treatment, but you buy certainty that you never pay tax on a gain that later evaporates. And where AMT was paid in a prior year, it is not lost: it becomes a minimum-tax credit recoverable in later years once your regular tax exceeds your tentative AMT (section “Alternative Minimum Tax: Understanding and Navigating Its Impact”).

The 90-day disqualification trap. ISO status is fragile on departure. Vested ISOs exercised more than three months after you leave ( IRC §422(a)(2); most plans implement this as a 90-day window, and one year if you separated due to disability) automatically convert to NSOs: the spread becomes immediately taxable as ordinary income and the AMT-free treatment is gone. The standard post-termination exercise window (section “What-If Scenarios and Financing”) is therefore not just a use-it-or-lose-it deadline on the options themselves — for ISOs it is also the deadline to preserve their tax character. Stacked on top of a large exercise cost for illiquid stock, this is where departing employees most often forfeit grants outright.

The capital-gains prize: QSBS. The reason to exercise and hold despite all of this is IRC §1202, “Partial exclusion for gain from certain small business stock”. Gain on stock in a qualifying domestic C-corporation, held long enough, can be up to 100% excluded from federal tax, capped at the greater of $15 million or ten times your basis. Exercising early — ideally early-exercising and filing the IRC §83(b) election while the spread is near zero — starts both the long-term and the QSBS holding clocks at the lowest possible cost. The full conditions, the OBBBA tiered holding periods, and the traps (the C-corporation requirement, the gross-asset ceiling, the unsettled SAFE/convertible-note clock) live in section “Qualified Small Business Stock”. For an early employee this is the difference between a 0% and a 23.8% federal rate on the exit — the single largest lever in the entire equity-compensation discussion.

The state does not forget where you vested. Equity compensation is sourced to where the services were performed, not where you live when the cash arrives. California allocates the income on an RSU vest by the fraction of workdays spent in California between grant and vest, and the spread on an NSO exercise by the California fraction between grant and exercise (FTB Pub. 1004) — so an employee who earned four years of grants in San Jose and moves to Texas the month before the IPO still owes California tax on nearly all of the vest-date income, reported on a nonresident return, years after leaving. New York runs the same allocation. None of this is reached by the Pension Source Tax Act (4 U.S.C. §114) shield that protects retirement income after a move (section “Domicile and the Conversion Year”): that statute covers qualified plans and IRAs, and equity compensation sits squarely outside it. The domicile play that works for a Roth conversion does not work for a vesting schedule. The levers that remain are the workday allocation itself — documented, because the burden of proving out-of-state days is yours — and the timing of new grants, which begin sourcing to your new state once you work there.

The full reporting mechanics — the AMT adjustment on Form 6251, basis corrections on Form 8949, the W-2 box 12 code V for NSO spreads, and the year-end timing moves that spread a large exercise across brackets — are detailed in section “Taxation of Incentive Stock Options (ISO)”, section “Taxation of Nonstatutory Stock Options”, and section “Year-End Planning With Stock Compensation”.

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