Schedule D and 1099-B: When Basis Doesn’t Match

Your 1099-B is the IRS’s window into your brokerage sales. Brokers report proceeds reliably; cost basis is where taxpayers and software stumble. When Schedule D and Form 8949 do not match what the broker sent to the IRS, you may get a CP2000 or an examiner letter. Fixing this is almost always a document reconciliation, not a debate about tax law.

TL;DR

Proceeds on 1099-B Box 1d should match your 8949 unless you have a documented adjustment.
Cost basis in Box 1e may be blank or wrong for uncovered lots, transfers, or equity comp shares.
Wash sale adjustments appear in Box 1g and related sections; your 8949 must reflect them.
Keep supplemental broker statements—not just the one-page 1099-B PDF.

Why basis differs: the usual suspects

1. Uncovered lots

If you transferred stock from another broker without transferring cost basis, the new broker may report $0 or incomplete basis in Box 1e. Your real basis lives in old trade confirmations or the prior broker’s cost basis report.

2. RSUs and ESPP

For restricted stock, ordinary income often appears on your W-2 when shares vest. Your 1099-B may still show $0 cost basis for the sale. Your adjustment belongs on Form 8949 with code B (or the code your software selects) and an explanation of W-2 income already taxed.

Document proof callout: Tie W-2 Box 1 (includes vest income) to per-share cost basis on the equity plan statement, then to 8949 rows for the sale.

3. Wash sales

If you repurchased the same stock within the wash sale window, the broker defers loss to a new lot. Your 1099-B may already adjust proceeds and basis; your return must match broker totals for the year.

Step-by-step reconciliation

Download the IRS copy of 1099-B and the broker’s realized gain/loss report for the full year.
Sum proceeds and cost basis by short-term vs. long-term buckets.
For each line on 8949, ensure codes in column (f) and adjustments in column (g) match the broker’s logic.
If you disagree with the broker, fix broker data first when possible; otherwise attach a statement with trade confirmations.

Worked example: $0 basis on RSU sale

Vest: Ordinary income $50,000 included in W-2 wages.
Sell: 1099-B shows proceeds $52,000, cost basis $0.
Correct: Basis $50,000 (cost of acquiring shares at vest) plus any broker fees, with adjustment code B per instructions.

FAQ

Do I need to attach statements to Form 8949?
Usually no if numbers match broker reporting. If you deviate, keep a PDF ready for the IRS and a short explanation in the software.

What if I have dozens of trades?
Use totals if allowed under IRS rules for your situation, or attach a broker statement that matches the 8949 summary.

How to act on this today

Upload your 1099-B and return to BasilTax and walk each 8949 line back to the broker PDF. If you are still trading, open an account and reconcile monthly so April is boring.

Educational content only—not individualized tax advice.