Keep the CSV, the ids, and the dollars. A lot file is not proof you identified the coins on time.
Not tax advice ยท last updated 25 October 2026
Bitcoin tax recordkeeping
Form 8949 asks for a description, dates, proceeds, and basis. You are the person who has to be able to show those figures. BasisFix only computes from the files you give it. The output is a review with a tax pro. IRS Publication 544 discusses how specific identification works for stock. This page does not decide whether that stock procedure applies to bitcoin.
What the worksheet can use
- The exchange or wallet CSV, including the transaction id when the file has one.
- The UTC timestamp the parser could read. A named timezone abbreviation is converted. An Excel serial date is rejected.
- The quantity and the USD amount the file states. A blank amount is not filled with a price feed. There is no price feed.
- Both sides of a move between your own accounts, so a shared id or a single matching amount can keep the original basis and date. See self-transfers.
The specific-identification JSON file is a calculation input. A lot id such as lot- plus the source and the external id tells the worksheet which lot to consume. It is not a record that you identified those units at the time of the sale. Keep that record yourself. The rules BasisFix applies when the file is present are on specific identification and picking lots before year-end.
How to keep records for this worksheet
- Keep the exchange CSV and the transaction id for each sale.
- Keep the USD amount you paid, including fees, and the date the file states.
- If you move coins, keep both the send and the receive so a later worksheet can match them.
- If you use specific identification, keep your own note of which units you sold when you sold them.
- Keep the worksheet with the label review with a tax pro. This page is not tax advice.
Questions
Is the specific-identification JSON proof of timing?
No. It tells this worksheet which lots to consume. It is not evidence that you identified those units when you sold them. Keep that record yourself.
What if the CSV has no dollar amount?
BasisFix leaves the amount blank. It does not download a price and it does not substitute zero. The sale that needs that basis stays out of the gain total.
Does Publication 544 settle bitcoin identification?
Publication 544 discusses specific identification for stock. This page does not decide whether that procedure applies to bitcoin. The worksheet still needs a file if you want it to use named lots.