When you swap tokens on an automated market maker (AMM) DEX, your wallet gives the interface permission to use the input token if needed, then asks smart contracts to exchange it through one or more liquidity pools. The trade executes only if it meets its price and timing checks; otherwise, it can revert. A displayed quote is an estimate, not a promise that the same price will still be available when the transaction executes.
What happens when I swap tokens on a DEX?
DEX is a broad category, and not every DEX uses the same contracts or trading model. The steps below describe a common pool-based AMM swap, using Uniswap as a documented example. In this model, you trade against pooled token reserves rather than matching with a queue of individual limit orders. Uniswap’s developer documentation puts it simply: “When a user swaps one token for another, they trade against the pool’s reserves.” Uniswap Developers: How Uniswap Works
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- You select the tokens and amount. The interface estimates how much output the available liquidity could provide. In Uniswap v2’s constant-product example, the reserves follow
x * y = k. A trade changes the reserve balance, and a larger trade relative to pool depth generally has more price impact. Other protocols and pool designs can use different pricing curves. Uniswap Developers: How Uniswap Works - The interface finds a route. Your trade may pass through one pool or several, sometimes using an intermediate token. Uniswap says its app evaluates liquidity across v2, v3, and v4 and compares output after fees and network costs. That is a description of Uniswap’s app, not a universal DEX routing rule. Its app lets users inspect the pools traversed and any split in a route. Uniswap Support: What is a swap route?
- You approve the input token if required. If the protocol has not already been allowed to use the token, the wallet may ask you to approve access. An approval grants a contract permission to transfer a specified token amount; it does not perform the swap. Uniswap’s flow may include an onchain approval and a Permit2 signature, followed by a separate confirmation for the swap itself. Approval mechanisms differ across tokens, protocols, and interfaces. Uniswap Support: How to approve tokens on Uniswap
- You confirm the swap. In a direct AMM flow, your wallet submits a transaction requesting that the relevant contracts carry out the exchange. Depending on the trade, the transaction can include a minimum output for an exact-input swap, a maximum input for an exact-output swap, and a deadline. These limits constrain execution; contract details vary between protocol versions. Uniswap Developers: Router02
- The contracts check conditions and execute against pool state. If the checks pass, the pool reserves change and the output token is delivered. Because the trade changes the pool, its price changes too; the size of that effect depends in part on trade size and available liquidity. Uniswap Developers: How Uniswap Works
- The swap settles or reverts. If execution would breach a limit, miss a deadline, or fail another contract check, the swap may revert instead of completing at an unacceptable price. A revert does not necessarily mean the transaction had no network-cost implications; the amount and treatment depend on the chain and transaction flow.
What do price impact and slippage mean?
Price impact: the trade moves the pool price
Price impact is the effect your trade has on the pool’s price as it uses available liquidity. It is related to the trade’s size compared with pool depth: a large trade in a shallow pool generally has more impact than a small trade in a deep one. It is not the same as a change that happens while your transaction is waiting. Uniswap Developers: How Uniswap Works
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Slippage: the execution price changes after the quote
Slippage describes a price change between the quote or submission and execution, including movement while a transaction is pending. A slippage tolerance sets a boundary for the outcome the contract will accept. If the available execution falls outside it, the swap can fail. Increasing tolerance allows a wider range of prices; it does not guarantee a better trade. Uniswap Support: What is slippage?
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What fees are involved in a DEX swap?
There are distinct costs, and which ones apply depends on the pool, chain, and swap flow.
- Pool or protocol fee: Uniswap’s swap overview says the swap fee is awarded to liquidity providers. The exact rate depends on the pool and protocol details. Uniswap Developers: How Uniswap Works
- Network cost (gas): A direct onchain transaction uses the network’s resources, and its cost depends on the chain and conditions at the time. Uniswap’s failure guidance says your wallet needs enough of the network token to cover network costs. Uniswap Support: How to fix a failed transaction
- Route-related costs: A route through multiple pools can involve different pool fees and network costs than another route. Compare the expected output after those costs, not just the headline token price. Uniswap Support: What is a swap route?
There is no single reliable figure for a typical DEX swap cost or slippage across chains and trades. A displayed estimate is specific to the route and conditions at the time.
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Why did my swap fail?
A failed swap usually means a required condition was not met or the token or network did not behave as expected. Uniswap’s support page, updated July 28, 2026, lists insufficient network funds, deadline expiry, slippage limits, token fees, and unsupported token behavior among possible causes. Uniswap Support: How to fix a failed transaction
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Check before signing
- Confirm the token contract address, especially when token names or symbols could be confused.
- Review the route, expected output, and displayed price impact.
- Check the slippage setting and deadline; do not raise slippage automatically just to force a transaction through.
- For an approval, verify the spender and the amount of token access requested.
- Make sure the wallet has enough of the network token for applicable network costs.
Some tokens charge transfer fees or have unsupported behavior, but a wider slippage tolerance also permits a worse execution price. Uniswap cautions that information from third-party token-checking services may not be accurate. Uniswap Support: How to fix a failed transaction
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Who submits the transaction in a UniswapX swap?
UniswapX is a distinct signed-order flow, not the same direct AMM transaction described above. In its 2023 whitepaper, Uniswap describes orders signed offchain and then executed and settled onchain: fillers pick up orders and submit them to a reactor contract. The whitepaper says fillers pay gas on the user’s behalf and recover the cost through the execution price. Its stated goals around MEV and price improvement are the protocol’s design claims, not independent guarantees. UniswapX Whitepaper
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AMM pool prices can diverge from prices elsewhere, creating arbitrage opportunities. Researchers have also studied transaction ordering and front-running around AMMs; a 2023 USENIX Security Symposium paper provides context on those mechanisms, but should not be treated as a current estimate of typical swap losses. USENIX Security Symposium 2023 paper
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