Got a CP2000 From a 1099-B? Cost Basis Is Usually Why

Here's a story that plays out thousands of times every year. You sold some stock during the year. Your brokerage sent you (and the IRS) a 1099-B showing the sale. You reported it on your return. Months later, a CP2000 arrives proposing that you owe tax on an amount that looks massively larger than any actual gain you made.

Don't panic. This is the single most common driver of CP2000 notices involving brokerage activity, and in most cases the fix is straightforward: send the IRS documentation of your cost basis.

What cost basis is, in plain English

Cost basis is what you paid to acquire a security. If you bought 100 shares of a stock for $50 per share, your cost basis is $5,000. When you later sell those shares, the taxable gain is the sale proceeds minus the cost basis.

Say you sell those same shares for $7,000. Your proceeds are $7,000, your basis is $5,000, and your taxable gain is $2,000. You pay tax on the $2,000 gain, not on the $7,000 of proceeds.

Cost basis is what makes the difference between reporting a modest gain and reporting a huge fake one.

The cost basis reporting problem

The IRS has required brokers to report cost basis for most transactions since 2011 (for stocks) and 2012 (for mutual funds). So in theory, the IRS should always know your basis.

In practice, there are several common scenarios where basis isn't reported to the IRS:

• Non-covered securities. Securities acquired before the cost basis reporting rules took effect. If you bought the shares in 2008 and sold them in 2022, the broker isn't required to report the basis to the IRS.
• Transferred shares. If shares moved between brokerages and the basis information didn't transfer cleanly, the receiving broker may not report basis to the IRS — even for "covered" securities.
• Incorrect basis on the 1099-B. The broker reports a basis, but it's wrong — usually because of a stock split, merger, return of capital, or some other corporate action that wasn't accounted for.
• Wash sales, options, short sales. Certain transactions have basis adjustments the broker may not track correctly.
• Crypto and collectibles. Most crypto exchanges and many alternative-asset platforms don't report basis at all.

When basis isn't reported, the IRS sees the gross proceeds and has no information to offset them. Their automated matching system treats the entire proceeds amount as potential taxable gain, and generates a CP2000 accordingly.

Why the proposed amount looks so huge

Here's the math that makes these notices so alarming. Say you sold $47,000 worth of stock during the year. You actually paid $43,000 to acquire it — a real gain of $4,000.

On your return, you correctly reported $4,000 in capital gain. But your broker reported only the $47,000 in gross proceeds to the IRS (because basis wasn't covered) and didn't report any basis. The IRS's matching system sees:

• Reported on return: $4,000 of income from stock sales
• Reported by broker: $47,000 of gross proceeds
• Difference: $43,000 of apparently unreported income

At a 22% marginal rate, that $43,000 "missing" amount turns into roughly $9,460 of proposed additional tax, plus potential accuracy penalty, plus interest. The notice lands in your mailbox showing you owe over $10,000.

In reality, you owe nothing additional. You reported the gain correctly. The IRS just doesn't know your basis.

What to do about it

The good news: this is one of the most clearly defensible CP2000 situations. You disagree, send documentation proving your cost basis, and the matter typically resolves.

Gather these documents:

• Your original 1099-B from the tax year in question — both pages, front and back, including any supplemental information pages
• Brokerage trade confirmations for the purchases that established your basis, if available
• Year-end brokerage statements showing cost basis, if available
• Transfer records if the shares were moved between brokerages
• A copy of your Form 8949 from the return, where you reported the sales
• A copy of Schedule D where the totals flowed

Many brokerages also provide a "Realized Gain/Loss Report" or "Tax Lot Detail" for each tax year that shows basis for every sale — even for non-covered securities. If your broker offers this, download it.

Writing the response

Your signed statement should be short and specific. Something in the shape of:

"The notice proposes $9,460 in additional tax based on $47,000 of stock sale proceeds reported on Form 1099-B. This amount was reported on my return, but with cost basis information that the broker did not transmit to the IRS. The actual cost basis for these transactions is $43,000, resulting in a net capital gain of $4,000, which is the amount reported on Form 8949 and Schedule D. Documentation supporting the $43,000 basis is attached."

Attach your documentation. Check "I don't agree" on the Response Form. Sign. Send via fax or certified mail.

If the shares were inherited or gifted

Basis rules get specific in two common scenarios:

• Inherited shares — basis is usually the fair market value on the date of the decedent's death (this is called "stepped-up basis"). This often eliminates most of the proposed tax, because the basis is close to the sale price.
• Gifted shares — basis carries over from the donor in most cases. If the donor's basis was low (they bought the shares decades ago), your basis is also low, and the gain could be substantial.

If your 1099-B involves inherited or gifted shares, document how and when you acquired them, and include that in your response.

When to involve a tax pro

For straightforward basis disputes — you have clean records and the math is simple — you can usually handle the response yourself. For more complicated situations, consider bringing in a CPA or enrolled agent:

• The proposed amount is substantial (typically $5,000+)
• There's a proposed accuracy-related penalty involved
• You're missing basis records and need to reconstruct them
• The transactions involve options, wash sales, or short sales
• The shares were acquired through employee stock plans, stock options, or RSUs — these have their own basis rules

A good tax professional can often completely eliminate the proposed amount in basis cases, which easily pays for their time.

The short version

Your 1099-B-based CP2000 almost certainly stems from missing or unreported cost basis. The IRS sees proceeds without basis and treats the whole amount as gain. Document your actual basis, explain the situation in a short signed statement, check "disagree" on the Response Form, and send everything together. Most basis-related CP2000 notices resolve completely once the IRS sees the documentation.

For the full walkthrough of how to structure your response, see our response guide.