A tax CPA opens her client's 1099-DA from Kraken. The form shows $100,000 in gross proceeds. Cost basis: $0. She knows the client bought those coins on Coinbase two years ago for $35,000, transferred them to Kraken, sold them there. The IRS sees a $100,000 gain. The reality is $65,000. The difference: $13,000 in phantom tax.

This scenario isn't hypothetical. It's happening to thousands of traders right now because of how broker-issued 1099-DA forms work. And because the IRS is beginning automated matching between 1099-DAs and filed returns, CPAs who don't reconcile these forms before filing season opens will miss the window to fix them cleanly.

Why Brokers Can't Report the Right Cost Basis

The problem sounds simple in theory but reveals a hard structural gap in how crypto exchanges operate.

When a client buys crypto on Platform A, that exchange knows the purchase price. When the client transfers coins to Platform B, the coins arrive but the purchase history doesn't. Platform B sees only arrival date and quantity. No record of original cost.

Now the client sells on Platform B. The exchange knows when coins arrived, when they sold, the sale price. It does not know the original purchase price. So it reports 1099-DA with cost basis at $0 or blank. It has no information to report.

The IRS loads the form into its matching system. When your client files their return, the computer matches reported gains to the 1099-DA. If the client correctly reports $65,000 in gain (using their own records), the IRS system sees $100,000 on the form and $65,000 on the return. Mismatch. Flagged. Notice issued.

Step What Happens Who Knows the Cost Basis?
1. Buy on Coinbase Client buys 1 BTC for $35,000 Coinbase only
2. Transfer to Kraken 1 BTC moves from Coinbase to Kraken Coinbase knows original cost; Kraken sees only arrival
3. Sell on Kraken 1 BTC sells for $100,000 Kraken knows sale price, not original cost
4. 1099-DA issued by Kraken Form reports $100k gross proceeds, $0 cost basis No broker has complete picture

The Scale of the Problem

This isn't rare. Analysis of 30,000 U.S. traders shows cost-basis mismatches are nearly universal for anyone trading across platforms. Average overstated gain: $14,500 per trader. Across the 2025 tax year sample, researchers found $435 million in inflated capital gains.

Now scale that to the entire U.S. trading population. And remember: IRS matching is automated and unforgiving. When the computer sees a mismatch, it generates either a CP2000 notice (proposed adjustment) or flags the return for human review. Both create work for your clients and require your response.

Timeline matters. Brokers issue 1099-DAs by early February. Clients begin filing late February. CPAs who spot mismatches in late March have lost the window to correct them before filing.

How CPAs Fix 1099-DA Mismatches with Form 8949

The IRS has a mechanism for exactly this situation: Form 8949, "Sales of Capital Assets." This form is where you reconcile broker-reported amounts against your actual cost basis.

Here's the workflow:

Step 1: Capture all transactions. Don't rely on the 1099-DA. Pull records from every platform your client used. List every purchase, transfer and sale with dates, amounts and cost basis. Include transfers between platforms. Include trades within a single platform (Kraken to Kraken) because those may also be missing cost basis.

Step 2: Identify the gaps. Line up your complete transaction history against the 1099-DA. Where does it match? Where does the broker form show zero cost basis? Where does the gain differ from your records? Document the discrepancies.

Step 3: File Form 8949 with correction codes. Code C means "correction: amounts on Form 1099-B/1099-DA do not match client records." Report the sale amount from the 1099-DA, then adjust cost basis to the correct amount. Show your client's actual gain or loss. Attach an explanation and file with the 1040.

The key: this tells the IRS you're aware of the 1099-DA and reporting the discrepancy intentionally with documentation. You're not hiding it. You're saying: "The broker reported this, but here's why it's wrong and what we actually owe tax on."

Why Timing Matters

If you file Form 8949 with a clear explanation before IRS matching begins, you're filing a complete, defensible return. If you file the return first (accepting the 1099-DA as-is) and then try to amend it later, you're fighting an automated system that has already logged the "mismatch." File the 8949 corrections with your original return, not as an amendment.