Updated July 28, 2026. Quick answer: Your basis in shares from a vest is their full value at vest — the amount already taxed as ordinary income on your W-2. Brokers frequently report $0 or a blank basis, and filing that as-is taxes the same dollars a second time.
Where the double tax comes from
At vest, the full share value is ordinary income on your W-2. That establishes your basis. When you later sell, gain should be measured from that basis — sell immediately and the gain is near zero.
But brokers commonly report the basis as $0, because the compensation element was handled by your employer’s payroll rather than the brokerage. Report the sale at that number and the entire proceeds look like gain, so you pay tax again on income you already paid tax on.
How to catch it
| Check | What you should see |
|---|---|
| 1099-B cost basis | Full fair market value at vest — not $0, not blank |
| Sold same day as vest | Gain or loss near zero, not a gain equal to proceeds |
| Supplemental broker statement | Usually shows the correct adjusted basis even when the 1099-B does not |
This is worth checking on returns you have already filed. It is one of the most common and most expensive equity-compensation filing errors, and it is correctable.
How the correction is made
The mechanism is an adjustment on Form 8949: report the proceeds and the basis as the broker gave them, then apply an adjustment to bring the basis to its correct figure, with the appropriate code. The supplemental statement your broker issues alongside the 1099-B usually contains the number you need.
Why same-day sales are the most commonly wrong
Sell-to-cover and same-day sales should produce almost no gain, because the sale happens at essentially the vest price. When you see a large gain on a same-day sale, that is the signature of a zero-basis report — it is the easiest version of this error to spot.
Sources
IRC §83(a) (income inclusion establishing basis); IRC §1012 (basis); Treas. Reg. §1.6045-1 (broker reporting of basis).
This states what the cited authority says. It is not tax advice, and equity compensation interacts with the rest of your return in ways a single page cannot see.