If you sold crypto through a U.S. exchange in 2025, a Form 1099-DA landed in your account this year. For most people it contains a surprise: Box 1f (Proceeds) is filled in, and Box 1g (Cost or Other Basis) is empty.
That looks like an error. It is not. But if you copy that form onto your tax return as-is, you can end up paying tax on the entire sale price instead of on your actual gain.
Why the basis box is blank
The IRS phased broker reporting in over two years:
- Sales effected on or after January 1, 2025 — brokers must report gross proceeds. They are not required to report basis information. (IRS, Instructions for Form 1099-DA, 2025)
- Sales effected on or after January 1, 2026 — brokers must also report basis for digital assets that are covered securities. For noncovered securities, basis reporting stays voluntary, and the broker checks Box 9.
So for the forms covering 2025 activity, blank basis is the expected default, not a mistake by your exchange.
Blank basis and zero basis are not the same thing
This distinction matters more than any other line in this article.
| What Box 1g shows | What it means |
|---|---|
| Empty | The broker did not report your cost. You supply your own cost. |
| 0 | The broker is asserting the asset actually had a basis of zero. |
The IRS instructions tell brokers to enter -0- in Box 1g only if the digital asset sold actually had a basis of zero. If you paid real money for the coin and your form shows a hard zero, that is worth questioning with the broker — a zero basis turns your entire proceeds into taxable gain.
What the 1099-DA does not know
A 1099-DA is a report of what one broker saw on its own platform. It does not know:
- transfers between your own wallets (these are not sales, but a broker only sees crypto arriving or leaving)
- what you paid when you originally acquired an asset elsewhere
- mining, staking, airdrop, or hard-fork income
- anything that happened in self-custody and never touched an exchange
- whether a transaction was long-term or short-term for you
The form is not a tax return, and it is not a substitute for your own records. You still report on Form 8949 and Schedule D, and you are responsible for the numbers you put there.
The practical fix, step by step
1. Collect every 1099-DA you received. One per broker. Line them up by asset, date, and units.
2. Export your own transaction history from every exchange and wallet you used — including the ones that never sent you a form. Smaller platforms, foreign exchanges, and self-custody wallets generally will not issue a 1099-DA, and those sales still belong on your return.
3. Match line by line. For each broker line, find the corresponding acquisition in your records. Four outcomes are possible:
- Matched — your record agrees with the broker on units and proceeds.
- Amount differs — same disposal, different numbers. Fees and transaction-cost treatment are the usual cause.
- Missing from your records — the broker reported a disposal you did not log.
- Missing from the broker’s form — you logged a disposal the broker did not report (common for transfers, DeFi, and non-U.S. platforms).
4. Rebuild the basis for every unmatched or blank line. You need an acquisition date, an acquisition cost, and a source you could point to if you were asked: an exchange CSV row, a bank transfer, an on-chain transaction hash.
5. Enter your cost in column (e) of Form 8949. When the broker simply left basis blank, you are not “correcting” a reported figure — you are supplying one that was never reported. Adjustment codes exist for correcting a basis the broker did report; they are not the tool for a blank box. If a broker reported a figure you believe is wrong, that is when an adjustment belongs on the form, and it is worth a conversation with a preparer.
6. Keep the working paper. Whatever you file, keep the reconciliation itself — the side-by-side of broker line versus your record, with the evidence reference for each basis number you supplied. That document is what turns “I think I paid about that” into something defensible years later.
Why this is harder than it sounds
Three things make crypto basis reconstruction messy in a way stock basis never was.
Per-wallet tracking is now mandatory. The final regulations ended the “universal” pool method. Basis is tracked wallet by wallet, account by account. Software that pooled everything for years may have consumed basis out of a wallet that never held the coin you sold.
Coverage is partial. You will get forms from some platforms and not others, which means your return is always a merge of broker data and your own data.
The record is old. Basis questions reach back years, through platforms that shut down, through wallets that were rotated, through spreadsheets nobody kept.
None of that is solved by the form. It is solved by a reconciliation.
A free tool for the side-by-side
We built BASISBRIDGE for exactly this step: put your own records next to every broker 1099-DA line, see where they disagree, rebuild cost basis wallet by wallet, and print a working paper that shows how each number was derived.
It runs as a single HTML file in your browser. No account, no upload, no installation — and it makes zero network calls, verified in testing across Chromium, Firefox and WebKit. Your tax data never leaves your computer, which matters in a year when crypto tax platforms have been breached.
- Free edition — up to 50 rows per table, enough to reconcile a straightforward year: BASISBRIDGE FREE
- Full edition ($49) — unlimited rows, FIFO/LIFO/HIFO lot methods, full evidence log: BASISBRIDGE
Do this before January
Reconciliation is much easier before the filing rush, and 2026 raises the stakes: for 2026 sales, brokers report basis for covered assets, and a broker figure that disagrees with your records is a much louder problem than a blank box. Getting your per-wallet history clean this year is the cheapest version of that work.
BASISBRIDGE is a working paper tool. It is not tax advice and it does not file a tax return. Talk to a qualified tax professional about your situation.
