Which swap records to keep for tax time

Tax authorities treat cryptocurrency swaps as taxable events in most jurisdictions. The moment you trade one token for another, you have crystallised a gain or a loss. Many people realise this only when they sit down with an accountant months later, and by then the swap confirmation screen is long gone. The page you are reading now explains what to save, why each piece matters, and what happens when you do not have it.

The core problem is that a swap is not a single event. It is a sequence of actions spread across at least two transactions, often on different chains, and sometimes involving multiple smart contracts. You send one token. A router splits your order across several liquidity pools. You receive a different token. The exchange site you used is a front-end; it may or may not store a record of your trade. The blockchain keeps the raw data, but raw data is not a tax record.

What actually happens during a swap

When you initiate a swap, your wallet sends an approval transaction to the token contract. This gives the swap router permission to spend your tokens. The approval is a transaction. It has its own hash, its own timestamp, its own gas fee. Then the router executes the swap. That is a second transaction. It is the one that moves your tokens out and delivers the new tokens to your wallet. The second transaction is the one that matters for tax purposes, but the first transaction is useful evidence that you intended to trade.

Between the approval and the execution, the router interacts with one or more liquidity pools. If you are swapping on a chain like Polygon, the router may use a path that goes through three pools. Each pool charges a fee. The final amount you receive is the amount after all fees. The blockchain records the entire path, but it does not label it as a swap. It records a series of internal calls. To reconstruct what happened, you need the transaction hash of the swap execution, then you need to decode the internal calls.

What to save and when

The safest habit is to save everything before you close the swap confirmation screen. What information should I screenshot before closing a swap confirmation screen is a question that every new trader asks after the screen is gone. The answer is: the transaction hash, the exact amount you sent, the exact amount you received, the token addresses, the chain name, the date and time in your local timezone, and the name of the exchange site. A screenshot of the confirmation screen is the best single piece of evidence because it ties the on-chain data to the interface you used. Without it, you have to guess which router contract was involved.

If you did not take a screenshot, you still have options. How do I find a swap I made six months ago if I only have the transaction hash is a common problem. The hash is the key. Paste it into a block explorer for the chain you used. The explorer will show you the timestamp, the sender wallet, the recipient wallet, the token transfers, and the gas fee. If you know the hash, you can find the rest. The problem is that you might not remember which chain you used. If you have the hash, try it on the major block explorers for Ethereum, Polygon, Binance Smart Chain, and Arbitrum. One of them will recognise it.

Why the details matter to an accountant

An accountant needs to know the exact time of the swap because the price of the tokens at that moment determines the gain or loss. If you swapped on Polygon, the timestamp on the block is in UTC. Why would an accountant need the exact timestamp from a swap I did on Polygon is a question that usually comes up when the market moved sharply within the same day. The block timestamp is the only objective record of when the trade happened. Your accountant will use that timestamp to look up the fair market value of both tokens at that moment. If you give them a date but not a time, and the price moved 10% that day, they have to estimate. Estimators increase the risk of an audit.

If the exchange site you used no longer exists, you still have the blockchain. Can I prove I swapped tokens if the exchange site no longer exists is a question that worries people who used smaller front-ends. Yes, you can prove it, but only if you have the transaction hash. The blockchain does not depend on the front-end. The swap contract that executed the trade is still there. The block explorer will show you the token transfers. The problem is that you lose the interface context. You cannot prove which exchange site you were using, only that the swap happened. That is usually enough for tax purposes, so long as you can show the trade was a swap and not a gift or a payment.

What a failed swap looks like

Not every swap succeeds. What does a failed swap look like on Polygonscan and what should I save from it is a question that often arises when a trade fails due to slippage, insufficient liquidity, or a gas price that was too low. On Polygonscan, a failed transaction will show a status of "Fail" in red. The gas fee is still charged. The tokens are not sent. You should save the transaction hash, the error message, and the gas fee amount. The error message tells you why it failed. The gas fee is a deductible expense. If you do not save the hash, you have no record of the fee, and you cannot prove you attempted a trade that did not go through.

What the router keeps and what it does not

The decentralised swap router itself does not keep logs. How long do decentralized swap routers keep logs of my trades is a question that misunderstands how they work. The router is a smart contract. It does not have a database. It has no memory of your trade after the transaction is complete. The only record is on the blockchain. The front-end you used may have kept logs, but that depends on the site. Some front-ends keep nothing. Others keep logs for a few months. You cannot rely on them. The blockchain is the only permanent record.

Receipt versus transaction record

There is a difference between a swap receipt and the on-chain transaction record. What is the difference between a swap receipt and the on-chain transaction record is a question that gets to the heart of the problem. A swap receipt is a document generated by the exchange site. It shows the trade details in a human-readable format. It may include the site's name, the date in your local time, and the exchange rate. The on-chain transaction record is the raw data on the block explorer. It shows the token transfers, the gas fee, and the block timestamp. Both are useful. The receipt is easier to read. The on-chain record is more authoritative. If they disagree, the on-chain record wins.

Matching chains

If you swap across chains, the recordkeeping becomes more complex. How do I match a swap I made on one chain to the tokens I received on another is a question that arises when you use a bridge or a cross-chain router. The swap on the source chain generates a transaction hash. The bridge sends a message to the destination chain, which then mints or releases the tokens. That second action has its own transaction hash. You need both hashes. The bridge contract may emit a log that links the two, but you cannot rely on the front-end to show you that link. Save both hashes. Save the source chain name and the destination chain name. Save the token addresses on both chains. Without both halves, you have an incomplete record.

What cannot be undone

If you lose the transaction hash, you cannot recover it from the blockchain. You can search your wallet history on a block explorer by entering your wallet address. That will show every transaction your wallet has ever made. You can scan through them manually. For a wallet that has made hundreds of swaps, that is tedious but possible. If you lose the wallet itself, the records are gone. There is no central authority that can restore them. The only backup is what you saved yourself.