Every windfall arrives with a silent partner. You see the gross number first and the government’s share much later.
Most people learn this with a bonus. Your employer withholds a flat percentage, the money lands, and you spend some of it. Months later a smaller refund explains what really happened.
That lesson gets expensive when the windfall is stock instead of cash. Equity does not carry a single tax rate. What you keep depends on what you hold, how long you have held it, and whether the sale counts as compensation or as an investment gain.
The rules also moved this year. One change that took effect at the start of the year makes a common equity decision cost more than the same decision cost last year, and the deadline to deal with it lands on Sept. 15.
Roughly $7 billion just landed in that gap.
OpenAI completed a secondary share sale totaling about $7 billion on Aug. 10, allowing current and former employees to sell stock at the company’s $852 billion valuation, according to CNBC.
The company bought the shares back with its own cash instead of bringing in outside investors, according to Bloomberg. It follows a $6.6 billion tender at a $500 billion valuation in October 2025 and a $1.5 billion offer in 2024.
The valuation held flat this time, the first of these deals that did not step the price up.
Why a tender offer is not the same as a paycheck
A tender offer is a company-run window in which employees sell some of their shares at a set price. For anyone paid mostly in equity, it is often the only chance to turn paper wealth into cash before an initial public offering, or IPO.
That structure is why the tax outcome is not uniform. Your neighbor at the same company, selling the same dollar amount on the same day, can keep a very different share of it.
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The reason is the instrument. Vested shares held more than a year can qualify for long-term capital gains treatment, generally capped at 20% federally, plus the 3.8% net investment income tax.
Options you never exercised are different. The spread between your strike price and the sale price is generally compensation, taxed as ordinary income at rates up to 37%.
“Two people selling the same dollar amount of equity can walk away with materially different amounts,” said Slice Global Equity CEO Maor Levran, whose firm handles equity and tax compliance for companies with employees in multiple countries.
What the 2026 alternative minimum tax reset changes for options
The more expensive change sits inside the alternative minimum tax, a parallel calculation that ignores several deductions the regular system allows and adds back items it does not tax. Exercising an incentive stock option and still holding the shares at year end is one of those add-backs.
Here is what changed, and what it costs:
- The AMT exemption phaseout now starts at $500,000 of alternative minimum taxable income for single filers and $1 million for joint filers, down from $626,350 and $1,252,700 in 2025, the instructions for IRS Form 6251 noted.
- The phaseout rate doubled to 50 cents on the dollar from 25 cents, so the exemption disappears twice as fast, according to the Tax Foundation‘s reading of IRS Revenue Procedure 2025-32.
- The 2026 exemption is $90,100 for single filers and $140,200 for joint filers under that same procedure, and it is gone entirely at $680,200 and $1,280,400.
- Employers may withhold a flat 22% on supplemental wages up to $1 million in a calendar year, with a mandatory 37% on anything above that, according to IRS Publication 15.
- Third-quarter estimated tax payments for 2026 are due Sept. 15, the IRS confirmed.
Read those together and the trap is visible. An option exercise is priced off the company’s current valuation, and the bigger that spread, the more gets added to your alternative minimum taxable income. That happens at exactly the moment the exemption shielding that income has been cut back.
“The biggest mistake is looking only at the eventual tax rate and ignoring the cash required today,” Levran said. “You can end up paying a significant AMT bill to hold shares that you still can’t sell.”
For anyone who vested across two countries, the arithmetic gets harder. Both places may claim the same proceeds depending on where the work was performed, and treaties and foreign tax credits reduce double taxation without erasing the reporting problem. An employer’s payroll allocation, Levran said, is not necessarily the final tax answer.
How to size your tax gap before the September deadline
None of this is unique to one company. It is the shape of an entire liquidity wave.
SpaceX employees got the public-market version of the same lesson. The company priced its initial public offering at $135 a share on June 11 and began trading the next day.
More Artificial Intelligence:
- David Tepper cuts AI stock, yet value hits $1.1 billion
- SpaceX, Google, Meta position themselves as the best of AI’s rest
- Jim Cramer reveals 6 AI stocks to watch in 2026
About 911.5 million insider shares became sellable on Aug. 6, by which point the stock had fallen more than 50% from its mid-June high, according to CNBC.
Those employees got liquidity and a moving price. OpenAI’s sellers got a fixed price and the same tax questions.
The practical work is the same either way. Find out what you actually hold, since a converted share, a restricted stock unit, and an unexercised option produce three different answers on identical proceeds.
Confirm the rate your employer withheld rather than assuming it covers the bill. Run the alternative minimum tax math before you exercise anything, not after.
The 22% that came out of your payout is a withholding convention, not your tax rate. The difference comes due on Sept. 15 or in April 2027, and it is far easier to find in a brokerage account now than in a checking account then.
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