You Got a CP2000 Notice About Your 1099-B — Here's Exactly How to Respond

It's a Tuesday in October, months after you filed, and there's an envelope from the IRS in your mailbox. Inside is a CP2000 notice claiming you underreported income and now owe thousands of dollars — often because of stock sales on a 1099-B the IRS says you didn't report correctly. Your stomach drops.

Take a breath. A CP2000 is not a bill and not an audit. It is an automated proposal generated when the income reported to the IRS by third parties (your broker, in this case) doesn't line up with what showed up on your return. And when the trigger is a 1099-B, the proposed balance is almost always dramatically overstated — because the IRS's computer assumes your cost basis is zero. This guide explains why that happens and gives you a step-by-step response that, in most 1099-B cases, reduces the amount owed to a fraction of the scary number on page one — or to nothing at all.

What a CP2000 Actually Is

The IRS runs an Automated Underreporter (AUR) program that matches every 1099, W-2, and 1098 it receives against your filed return. When something doesn't match, it generates a CP2000: Notice of Proposed Adjustment for Underpayment/Overpayment. Key facts to anchor on:

• It is a proposal, not a final determination. You have the right to disagree.
• You typically have 30 days from the notice date to respond (60 if you're outside the U.S.).
• It is not an audit and does not become one if you respond correctly.
• The proposed tax, penalty, and interest on the notice assume the IRS's version of events is right. For stock sales, that version is almost always wrong in your favor.

Why 1099-B CP2000s Are Almost Always Overstated

Here's the mechanism that catches so many investors. When your broker reports a sale to the IRS on a 1099-B, it reports the gross proceeds — the full amount you received. For many transactions, especially "noncovered" securities, the broker does not report your cost basis to the IRS.

The AUR system sees proceeds with no basis, and it does the only thing it can: it treats your entire proceeds as taxable gain. If you sold $80,000 of stock that only earned you a $3,000 profit, the CP2000 may propose tax on the full $80,000 as if it were pure gain. That's how a modest trading year turns into a five-figure phantom tax bill.

The reason this happens to you specifically is usually one of these:

• You didn't report the sales at all (forgot a brokerage account, thought a small account didn't matter).
• You reported them but the IRS couldn't match them (summary reporting without an attached Form 8949, or a broker name mismatch).
• You reported proceeds but the basis wasn't visible to the AUR system in a way it could reconcile.

In every one of these cases, the fix is the same: show the IRS your actual cost basis.

Step 1: Read the Notice and Find the Response Date

Turn to the response page. Note the deadline and the notice number (CP2000) and the tax year in question. Everything you send back references these. Do not ignore it — if you miss the deadline, the IRS issues a Statutory Notice of Deficiency (CP3219A) and the proposed amount becomes much harder to unwind.

Step 2: Reconcile the 1099-B Line by Line

Pull the 1099-B(s) for the year in question and rebuild the numbers the way they should have appeared on Form 8949 and Schedule D. For each sale you need four things:

1. Description of the security
2. Proceeds (this should match what the IRS has)
3. Cost basis (this is what the IRS is missing)
4. Gain or loss (proceeds minus basis, with any adjustments)

Total these up. Your real gain is proceeds minus basis — not proceeds alone. If you can't find basis for older lots, our guide to recovering missing cost basis on noncovered securities shows where to look (broker transaction history, old statements, DRIP records, corporate action notices).

If part of the mismatch was actually a wash sale or an adjustment the IRS didn't see, the same reconciliation applies — this is the exact process from our 1099-B doesn't match Schedule D walkthrough, just done after the fact.

Step 3: Decide Whether You Agree or Disagree

The CP2000 gives you three paths:

• Agree — if the IRS is right and you did underreport, sign the response form and arrange payment.
• Partially agree — common with 1099-Bs: you agree the sales were unreported but disagree with the amount, because the real gain is far smaller once basis is included.
• Disagree — you already reported everything correctly and the IRS mismatched it (e.g., you used summary reporting).

For most 1099-B notices, you'll partially agree: yes, these sales happened; no, the tax isn't on the gross proceeds — here's the basis.

Step 4: Assemble Your Response Package

Send back a clean, complete package. Include:

1. The CP2000 response page, checkbox marked (agree / disagree), signed and dated.
2. A corrected Form 8949 showing every sale with proceeds, cost basis, and gain/loss. This is the single most important document.
3. A corrected Schedule D summarizing the 8949 totals.
4. A short cover letter — one page, plain language: "The proposed adjustment does not account for my cost basis. The enclosed Form 8949 shows actual proceeds of $X, cost basis of $Y, and a net gain of $Z. The corrected tax owed is $, not the proposed $."
5. Supporting documentation for basis if you have it (brokerage statements, purchase confirmations).

Do not file a Form 1040-X amended return in response to a CP2000 unless the IRS specifically tells you to — the CP2000 response is the correction channel. Filing both can create a duplicate-processing mess.

Step 5: Send It the Right Way and Keep Proof

Respond by the deadline using the address or fax number on the notice (not the general IRS filing address). Send by certified mail with return receipt, or fax and keep the confirmation. Keep a full copy of everything. Processing can take 8–12 weeks or longer, so don't panic if you don't hear back quickly. If you're close to the deadline and need more time, you can call the number on the notice to request an extension.

What Happens Next

If your reconciliation is accepted, the IRS sends a CP2005 ("we've closed our review, no change" or "adjusted as you proposed"). If they partially accept, you'll get a revised balance far below the original. If there's still a gap, you can continue to correspond or request an appeal — you have rights all the way up the chain. The key is that responding on time with a documented basis almost always shrinks the number dramatically.

FAQ

Is a CP2000 an audit?

No. It's an automated notice proposing a change based on document matching. It doesn't become an audit if you respond, and responding correctly usually closes it.

Why does the IRS think I owe so much on my stock sales?

Because the 1099-B reported your proceeds but not your cost basis, so the IRS treated the entire sale amount as gain. Once you supply the basis, the real gain — and the tax — is far smaller.

Do I need to file an amended return (1040-X)?

Usually no. The CP2000 response form is the correction mechanism. Only file a 1040-X if the notice explicitly instructs you to, or if there are other unrelated changes to your return.

What if I can't find my cost basis?

Reconstruct it from brokerage transaction history, old statements, or the broker's online records. For inherited or gifted shares, basis is set by special rules. If you genuinely cannot document it, you must still make a good-faith estimate rather than accept a zero-basis proposal.

What happens if I ignore the CP2000?

The IRS issues a Notice of Deficiency (CP3219A), and the proposed amount can become a legally assessed balance. Always respond by the deadline, even if only to request more time.

Can this happen even though I used tax software?

Yes. If you used summary reporting without transmitting the detail, or an account didn't import, the AUR system may not have matched your sales — even though your return was substantively correct.

Bottom Line

A CP2000 tied to a 1099-B looks terrifying because the proposed balance assumes your cost basis is zero. It almost never is. Your job is to reconcile every sale — proceeds, basis, gain — onto a corrected Form 8949, mark the response form, write a one-page cover letter, and mail it back before the deadline with proof of delivery.

Do that, and the phantom tax bill usually collapses to the small real number it should have been all along. The mistake was never that you owed a fortune — it was that the IRS couldn't see what you paid.

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