Form 8949 Attachment: Format, Filing, and Recordkeeping

A Form 8949 attachment is a statement listing every individual capital asset sale, filed alongside Form 8949 so you only have to enter combined totals on the form itself. The IRS allows this under what the instructions call Exception 2, and there is no minimum number of trades required to use it. What matters is that the detail on your statement reconciles cleanly with the totals you report and with the 1099s the IRS already has on file.

When You Actually Need an Attachment

Before assembling anything, check whether you can skip Form 8949 altogether. Under Exception 1, you can report qualifying trades directly on Schedule D (line 1a or 8a) with no Form 8949 and no attachment, if every one of the following is true for that group of transactions:¹

Your broker reported cost basis to the IRS on Form 1099-B or Form 1099-DA.
The form shows no adjustments.
The “Ordinary” box on the 1099-B or 1099-DA is not checked.
You don’t need to correct the basis, proceeds, or type of gain or loss.
You aren’t deferring gain through a Qualified Opportunity Fund.
The transactions don’t involve collectibles.

If any transaction fails one of those tests, that transaction needs Form 8949 treatment, and if you have more than a handful of them, the attached statement is the practical way to file. Trades that fit Exception 1 and trades that don’t can coexist on the same return: use the shortcut for the clean ones and the attachment for the rest.

What Goes on Form 8949 Itself

When you file an attachment, Form 8949 becomes a summary sheet. For each box that applies to your situation, you enter one row per broker with the following:¹

Column (a): the broker’s name followed by “see attached statement.”
Columns (b) and (c): blank.
Column (f): the code “M,” plus any other adjustment codes that apply to transactions within that group.
Columns (d), (e), (g), and (h): combined totals for proceeds, basis, adjustments, and gain or loss from the corresponding section of your statement.

Each broker gets its own summary row within each box. Lumping multiple brokers into a single line is the most common way people generate a mismatch notice, because the IRS reconciles your totals against the 1099s it received from each firm separately.

Form 8949 is divided into short-term (Part I) and long-term (Part II), and within each part the box you check tells the IRS how the trades were reported to you: basis reported on a 1099-B, basis not reported on a 1099-B, no 1099-B received, and three parallel categories for digital assets on Form 1099-DA.³ The attachment either accompanies the return or gets mailed with Form 8453; there is no third option where the IRS holds off until they ask.

Building the Statement

Your statement has to carry the same fields as Form 8949 and follow a similar layout. For each transaction, that means:¹

Description of property (for example, “100 sh XYZ Corp” or “2.5 BTC”)
Date acquired
Date sold or disposed of
Proceeds
Cost or other basis
Adjustment code and amount, if any
Gain or loss

The IRS doesn’t dictate a file format. A spreadsheet exported to PDF is the standard approach. What matters is the organization: group transactions by their box category, and give each group its own subtotal for proceeds, basis, adjustments, and gain or loss. Those subtotals are the numbers that go on Form 8949.

Don’t mix short-term with long-term. Don’t mix 1099-B transactions with 1099-DA transactions. Someone reviewing the return should be able to trace any single line item back to the summary row it feeds.

Reconciling With the 1099

The IRS receives a copy of every Form 1099-B, so the numbers on your statement need to match the broker’s or explain the difference explicitly.⁴ Adjusting a broker-reported basis without showing an adjustment code and amount is the single fastest way to draw a notice.

For digital assets, brokers started issuing Form 1099-DA for gross proceeds on sales in 2025, and cost basis reporting for covered digital asset securities begins with 2026 sales.⁵ If the exchange didn’t report basis, or if you moved tokens between wallets before selling, you have to calculate and document basis from your own records. Those trades land in the “basis not reported” or “no 1099-DA received” boxes.

Adjustment Codes

When transactions in your statement need adjustments, each one carries a code in column (f) of the statement explaining why. The codes you’ll see most often:⁶

Code B: basis on the 1099 is incorrect; enter the correction in column (g).
Code W: loss disallowed under the wash sale rule; enter the disallowed portion as a positive adjustment in column (g).
Code M: multiple transactions summarized on one row. This is the code that flags an Exception 2 filing on Form 8949 itself.
Code E: selling expenses or transaction costs not already reflected on the 1099.
Code H: excludable gain from the sale of a main home.
Code T: type of gain or loss on the 1099 is wrong (for example, reported short-term when it should be long-term).

More than one code can apply to a single transaction. A wash sale with a bad basis carries both “W” and “B,” and the amounts in column (g) are combined.

Digital Assets Go in Their Own Section

Cryptocurrency and other digital assets follow the same process but must sit in their own box categories rather than mixed in with stocks and bonds.⁶ If a single attachment covers both traditional securities and digital assets, keep them in separate sections of the file with separate subtotals feeding separate summary rows on Form 8949.

How to Submit the Attachment

E-Filing

Most tax software accepts a PDF of the statement that transmits with the electronic return. Individual PDF files need to stay under 60 MB, the total of all attachments on the return can’t exceed 1 GB, and the files can’t be password-protected.⁷ Use a descriptive file name (something like “Form8949_Attachment_ShortTerm”) and keep it under 64 characters.

If your software can’t handle the attachment, or you’d rather send the detail on paper, there’s a hybrid option: e-file the return, then mail Form 8453 with the Form 8949 statement after the electronic return is accepted.⁸

Paper Filing

Mailing the return means physically attaching the statement behind Form 8949. Put your name and taxpayer identification number on every page.¹ Pages get separated during processing, and a page without identifying information can’t be reconnected to your return.

Keeping the Records

Hold onto the broker statements, the prepared attachment, and a copy of the filed return for at least three years from the filing date. If you underreport gross income by more than 25%, the IRS has six years to assess additional tax, so six years is the safer practice for anyone with significant capital gains.⁹

Internal Revenue Service. Instructions for Form 8949 – Section: Exceptions to Reporting Each Transaction on a Separate Row

Internal Revenue Service. Form 8949 (2025)
One prohibition worth flagging: don’t write “Available upon request” on Form 8949 in place of the detail. The instructions specifically forbid it.2Internal Revenue Service. Instructions for Form 8949 – Section: Exceptions to Reporting Each Transaction on a Separate Row

Internal Revenue Service. About Form 1099-B, Proceeds from Broker and Barter Exchange Transactions

Internal Revenue Service. Instructions for Form 1099-DA (2025)

Internal Revenue Service. Instructions for Form 8949 – Sales and Other Dispositions of Capital Assets

Internal Revenue Service. Recommended Names and Descriptions for PDF Files Attached to Modernized e-File (MeF) Business Submissions

Internal Revenue Service. Instructions for Form 8949 – Section: E-file

Internal Revenue Service.