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When you swap tokens on an automated market maker (AMM) DEX, the app finds a route through one or more liquidity pools, and a transaction asks smart contracts to trade against those pools. Your wallet may first need to approve the input token. The swap then executes only if its price and timing checks pass; otherwise, it can revert. The exact steps depend on the DEX, blockchain, route, and swap design.
What happens when I swap tokens on a DEX?
On an AMM, you trade against liquidity held in pools rather than matching with a specific person’s limit order. Uniswap’s developer documentation puts it simply: “When a user swaps one token for another, they trade against the pool’s reserves.” The reserves and the pool’s pricing rules determine the quote and affect the price at which the trade executes. Uniswap Developers: How Uniswap Works
- You choose the tokens and amount. The interface estimates how much of the output token you could receive using available liquidity. In Uniswap v2’s constant-product example, pool reserves are related by
x * y = k. Trading a larger amount relative to a pool’s depth generally causes greater price impact. Other protocols and pool designs can use different pricing curves. Uniswap Developers: How Uniswap Works Uniswap Developers: Swaps - The app selects a route. A route might use one pool or pass through intermediate tokens and multiple pools. Uniswap says its app evaluates liquidity across v2, v3, and v4 and compares output after fees and network costs; that describes Uniswap’s routing, not every DEX’s method. Its app lets you inspect the pools used and any split in the route. Uniswap Support: How does Uniswap find the best price for swaps?
- Your wallet may request token approval. If the relevant contract has not already been given permission to use the input token, the interface may ask you to approve access. That approval is permission for a contract to spend tokens up to an allowance; it is not the swap itself. Uniswap’s flow can involve an onchain approval and a Permit2 signature. You still need to confirm the swap separately. Approval mechanisms vary by token, protocol, and interface. Uniswap Support: What is an approval transaction?
- You confirm the swap. In a typical direct AMM flow, your wallet submits a transaction asking the relevant contracts to execute the trade. The transaction can include protective conditions: a minimum output for an exact-input swap, a maximum input for an exact-output swap, and sometimes a deadline. Contract details differ across protocol versions, so the implementation of one DEX or pool should not be assumed to apply to all others. Uniswap Developers: Swaps
- The contract checks the trade against current pool state. If its conditions are met, the trade changes pool reserves and the output token is delivered. The quote shown before confirmation is an estimate for a possible execution, not a guarantee that the market or pool state will remain unchanged while the transaction is pending.
- The trade settles or reverts. A swap can revert if executing it would violate a minimum-output, maximum-input, deadline, or other contract condition. Network-cost consequences for a failed transaction depend on the chain and transaction flow.
What do price impact and slippage mean?
They describe different parts of the trade. Price impact comes from the size of your trade relative to available liquidity. Slippage describes a change in the price or output between the quote and execution—for example, because pool conditions changed while a transaction was pending. A slippage tolerance sets the boundary your swap will accept; if execution falls outside that boundary, the trade may fail. A wider tolerance permits a wider range of outcomes, including a worse execution. Uniswap Support: What is slippage?
Pool prices can also diverge from prices elsewhere, creating arbitrage opportunities. A 2023 USENIX Security Symposium paper discusses AMM structure and transaction ordering, including front-running. It provides context for how ordering can matter, but does not establish a current, market-wide rate of losses from MEV. USENIX Security Symposium 2023 paper
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What fees might I pay?
- Network costs: These pay for onchain transaction execution and vary with the blockchain and conditions at the time. For a direct onchain flow, your wallet needs enough of the network’s token to cover the cost. Uniswap’s failure guidance lists insufficient network funds as a possible reason a transaction cannot proceed. Uniswap Support: Why is my transaction failing?
- Pool or protocol fees: These are separate from network costs. Uniswap’s swap overview says swap fees are awarded to liquidity providers; the rate depends on the pool and protocol details. Uniswap Developers: Fees
- Other execution costs: The route and swap design affect the effective output. A route through multiple pools, for example, should be evaluated by its output after fees and network costs rather than by the displayed exchange rate alone.
Why did my swap fail?
Uniswap’s support page, updated July 28, 2026, lists insufficient network funds, an expired deadline, slippage limits, token fees, and unsupported token behavior among possible causes. The same page reports interface defaults of a 0.5% slippage limit and a 30-minute deadline. Those are dated Uniswap interface settings, not universal DEX defaults or recommendations; settings can change. Uniswap Support: Why is my transaction failing?
Before signing, check the following:
- Confirm the token contract address, especially if several tokens share a name or ticker.
- Inspect the route and pools used, if the interface shows them.
- Compare expected output and price impact with what you intend to accept.
- Review the slippage setting and any deadline; do not raise slippage automatically just to force a trade through.
- For an approval, verify the spender and allowance amount shown by the wallet.
- Check the network fee and ensure your wallet has enough of the required network token.
Some tokens impose transfer fees, which can affect a swap. A wider slippage tolerance may help accommodate certain token behaviors, but it also allows a worse execution. Uniswap’s support page cautions that information from third-party token-checking services may not be accurate.
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Is every DEX swap a direct pool transaction?
No. Uniswap is a useful example of an AMM, not a definition of every DEX. Uniswap itself distinguishes v2, v3, and v4 mechanics, and its UniswapX flow differs from a typical direct AMM transaction. Other DEXs may use different pricing, routing, approvals, or settlement.
Direct AMM swap
In the typical direct flow, your wallet submits an onchain transaction to the relevant contracts. The contracts execute against pool liquidity and apply the trade’s checks. Network costs are associated with transaction execution on the chain.
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UniswapX signed-order flow
UniswapX’s 2023 whitepaper describes a different sequence: a user signs an order offchain, then a filler picks it up and submits it to a reactor contract for onchain settlement. The whitepaper says fillers pay gas on the swapper’s behalf and recover that cost through the execution price. Its stated goals around MEV and price improvement are Uniswap’s design claims, not an independent guarantee of a particular result. UniswapX whitepaper
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