Decentralized exchanges: check a swap before you sign
A decentralized exchange (DEX) lets you trade through blockchain contracts instead of placing funds in a conventional exchange account. Keeping your keys changes the risks; it does not remove them. This lesson follows a simple same-network token swap, from checking the asset to reading the receipt.
Your task: explain what you are authorizing, the least you will receive and the total cost before approving a swap.
Follow the trade through three layers
Wallet: your authorization
Connecting shows the app your public address. An approval or permit can authorize token spending; a swap executes the trade. These are different actions. A signature without a gas fee can still grant valuable permissions. Never enter a recovery phrase into a trading website.
Router: the proposed route
An automated market maker trades against pool reserves; an order-book DEX matches orders. Aggregators may split a trade across several pools or use other execution systems. Read the route and its terms. A familiar interface does not make every token or contract trustworthy.
Blockchain: the settlement
A submitted transaction is pending until included. A successful receipt records execution, not whether you bought a legitimate asset. Smart-contract bugs, malicious tokens, compromised websites and transaction ordering remain risks. A same-network swap does not move assets to another blockchain.
Walkthrough: inspect the quote, then the wallet request
Verify the network and asset
Open the protocol from a verified official source. Match the network in the app and wallet. Compare the full token contract address with the project’s official documentation and a block explorer; a ticker or logo is not an identifier. Keep enough native gas currency for approvals, the swap and later transfers.
Read the complete quote
Check input, expected output, price impact, minimum received and every displayed fee. Price impact comes from your trade; slippage describes a change between quote and execution. Raising slippage tolerance does not improve a bad quote. Refresh an old quote and reconsider the amount or route if the result is unacceptable.
Authorize, then verify
Check the spending contract, token, allowance and any expiry in each wallet prompt. Limit permissions to what you understand. After submission, inspect the transaction hash on the correct explorer: status, token contract, amounts and recipient. Disconnecting the app does not revoke token approvals; review unused permissions separately.
Worked example: a minimum is a boundary, not a promise
Suppose a standard exact-input pool swap quotes 200 tokens for 100 USDC. A 1% tolerance gives an illustrative minimum of 198 tokens. Pool fees are already included in this hypothetical quote; gas is separate. If execution would deliver 197, a correctly enforced minimum makes the swap revert. An included failed transaction can still consume gas. At 5% tolerance the minimum becomes 190: you have allowed a worse result, not reduced price impact. Read the actual wallet or order minimum because execution systems can differ.
Three mistakes to catch before signing
Treating a ticker as proof
Two contracts can use the same name. Check the full address and network independently. A small successful purchase does not prove that you can later sell, or that the contract cannot change its restrictions.
Approving an unlimited allowance casually
A spending approval may expose more than this trade. Check what the spender can access, including signed permits. An audit or a hardware wallet does not make an unsafe authorization harmless.
Swapping the entire gas balance
Tokens on one network cannot pay fees on another merely because their tickers match. Leave the required gas asset available. Check a pending transaction before submitting again to avoid unintended repeat trades.
Try it: spot the unsafe decision
A quote shows 500 tokens, 2% tolerance and separate gas. You calculate a minimum of 490. The wallet requests unlimited spending by an unfamiliar contract. Should you sign because the minimum looks correct? What if the transaction reverts after inclusion?
Reveal the reasoning
No. The arithmetic checks only the output boundary, not the spending permission or token legitimacy. Stop and verify the spender and scope. If an included transaction reverts, its swap state changes are rolled back, but gas used can still be charged. The 490-token boundary does not protect against every other risk.
