> ## Content Index > Fetch the complete content index at: https://kintaronote.com/llms.txt > Use this file to discover other available public pages before exploring further. # Why Your DEX Swaps Cost More Than Expected: Understanding MEV and Sandwich Attacks - URL: https://kintaronote.com/why-your-dex-swaps-cost-more-than-expected-understanding-mev-and-sandwich-attacks/ - Published: 2026-09-09T02:13:40.000Z - Updated: 2026-09-09T02:13:40.000Z - Author: kintaro - Tags: DeFi ## Understanding MEV and Sandwich Attacks: What Happens When You Swap on a DEX You check the price, review the amount, and hit **Swap**. A few seconds later, the transaction is confirmed. The tokens are in your wallet. Nothing seems wrong. Then you look at the final numbers and notice that the price was worse than expected. That does not always mean someone attacked your trade. Crypto prices move constantly, and the amount of liquidity available in a trading pool can change from one moment to the next. But sometimes, **MEV** can be part of the reason. MEV stands for **Maximal Extractable Value**. In simple terms, it refers to opportunities to make money from the way blockchain transactions are selected, ordered, and included in blocks. Some of those opportunities are normal parts of crypto markets, while others can work against ordinary traders. One of the clearest examples is a **sandwich attack**. You do not need to understand the entire Ethereum network to grasp the basic idea. Once you know what happens before and after a DEX trade, the subject becomes much easier to follow. --- ## What Is a DEX? **DEX** means **decentralized exchange**. It allows people to trade crypto through blockchain-based smart contracts instead of sending an order to a traditional centralized exchange, with **Uniswap** being one of the most widely known examples on Ethereum. Many DEXs use **liquidity pools** rather than a traditional order book. A liquidity pool holds two or more tokens that traders can swap between. Take an ETH/USDC pool as a simple example. If someone buys ETH with USDC, the amount of ETH and USDC in the pool changes. Because the pool's balance changes, the price available to the next trader can change as well. That simple mechanism is one reason DEX trading can behave differently from trading on a traditional exchange. --- ## What Is an MEV Bot? An **MEV bot** is software that looks for profitable opportunities in blockchain activity and can automatically submit transactions when it finds one. You may also see the term **MEV searcher**, which can be an individual, a trading firm, or an automated program. The important part is the software's ability to watch transactions and react quickly. A regular trader might take several seconds to review a swap and approve it, but a bot does not have to stop and think about the decision. It can monitor many transactions at once, calculate possible price movements, and act when an opportunity looks profitable. That does not make every MEV bot malicious. **Arbitrage** is a good example, where a trader buys an asset where it is cheaper and sells it where it is more expensive, helping reduce price differences across markets. The concern for DEX users is narrower: some strategies can turn a user's pending transaction into an opportunity to make money at the user's expense. --- ## How a Sandwich Attack Works A sandwich attack puts an attacker's transactions around a user's transaction like two pieces of bread wrapping around the fillings, which is why it earned its name. The basic pattern looks like this: 1. The user submits a trade. 2. An attacker trades before the user. 3. The user's trade executes at a worse price. 4. The attacker trades again afterward. The idea is simple enough, but the details are worth looking at step by step. ### 1\. A User Submits a Swap Imagine a trader wants to buy a token on a DEX. The trader enters the amount, checks the quote, and submits the transaction. The transaction now has to make its way through the blockchain's transaction system before it becomes part of a confirmed block. ### 2\. A Bot Sees the Opportunity Depending on how the transaction is submitted and which network is being used, information about a pending transaction may be visible to other participants before confirmation. An automated system may notice that the trade is large enough to move the price of a liquidity pool. If the expected profit is high enough, the bot can submit its own transaction with the goal of getting it processed first—acting as the "top slice of bread" (front-running) to buy up the tokens and push the price higher. ### 3\. The Bot Trades First The bot buys the token before the user's transaction. That trade changes the pool's balance and can push the available price higher. The user's transaction then executes against the changed pool, resulting in fewer tokens than the trader expected from the original quote. ### 4\. The Bot Trades Again After the user's transaction, the bot can sell the tokens it purchased earlier as the "bottom slice of bread" in what is often called a **back-run**. The attacker has effectively put one transaction in front of the user's trade and another behind it, trapping the user's order right in the middle. --- ## Why Does This Matter to Regular Traders? The frustrating part is that a sandwich attack does not necessarily look like an attack. The transaction can succeed, the tokens can arrive, and the wallet can show everything as normal. The problem may only become obvious when the final execution price is compared with the quote shown before the trade. For a small trade, the difference may be difficult to notice. Across larger trades, or across many transactions, even relatively small price differences can matter—just like when traders rushing to buy a trending new meme coin suddenly realize they paid 5% more than expected because a bot slipped right into their transaction path. Automated systems also have an advantage that individual traders do not: they can watch markets continuously and respond much faster than a person working manually. --- ## What Does the Mempool Have to Do With It? The **mempool** is short for **memory pool**. It generally refers to the collection of submitted transactions waiting to be included in a confirmed block. It helps to think of the mempool as a waiting area, although not every blockchain or transaction system handles pending transactions in exactly the same way. When transaction information is visible before confirmation, other participants may be able to react to it. That is one reason transaction visibility and routing matter when talking about MEV. --- ## Slippage Does Not Automatically Mean MEV This is one of the easiest parts of DEX trading to misunderstand. **Slippage** is the difference between the price expected when a trade is submitted and the price received when it actually executes. There are plenty of reasons for slippage that have nothing to do with an MEV bot: - Market prices can move. - Liquidity can change. - A large order can move the pool's price. - Other traders can submit transactions at nearly the same time. There is also **price impact**, which is the effect of your own trade on the price available in a liquidity pool. If a pool is small and a trade is large, the trade itself can move the price significantly. So a worse execution price is not proof of a sandwich attack; it is better to look at the full situation before deciding what happened. --- ## Should Slippage Always Be Set to 0.1%? No. A very low slippage setting can sound like the safest choice, but it can create another problem: the transaction may fail. The slippage limit tells the DEX how much price movement the trader is willing to accept before the transaction should stop rather than complete. A tighter limit gives you less room for a worse execution price, but markets are not all equally stable or liquid. A highly liquid ETH/USDC market is not the same as a small meme-coin pool with limited liquidity. If the slippage setting is too tight for the market, the transaction can revert. If it is too loose, the trade may accept a price that is much worse than intended. There is no universal number that works for every swap. --- ## Why Low-Liquidity Tokens Need Extra Care **Liquidity** describes how much trading activity and available assets a market has to handle transactions. Deep liquidity generally means a market can absorb larger trades with less price movement, while thin liquidity is a different story. Suppose a token has a relatively small liquidity pool. A large buy can move the price much more than the same trade would move the price in a deep pool, meaning higher **price impact**. It can also create conditions that are more attractive to certain MEV strategies. This is why a slippage setting that works well for a major trading pair should not automatically be copied onto a low-liquidity token. --- ## Can Private Transaction Routing Help? This is where **private transaction routing** becomes relevant. The basic idea is to send a transaction through a route that keeps it out of certain public transaction flows before it is included in a block. If an attacker cannot see the pending transaction through the relevant public route, it becomes harder to build a sandwich around it. One well-known Ethereum example is **Flashbots Protect**, which provides infrastructure for interacting with the Ethereum MEV ecosystem and offers transaction-protection services designed to reduce certain forms of front-running exposure. Uniswap also provides swap protection for eligible Ethereum Mainnet transactions and describes it as protection against front-running and sandwich attacks. Private routing is not a magic shield and does not remove every risk from a DEX trade; it is better understood as a way to reduce a particular type of transaction exposure. --- ## What Is an RPC? The term **RPC** often appears in discussions about blockchain transaction routing and stands for **Remote Procedure Call**. In practical terms, an RPC endpoint is a connection that allows a wallet or application to communicate with a blockchain network. A wallet uses an RPC service to send transactions and request information from the network. Different RPC services can support different features and transaction-routing methods, and some can be connected to private transaction systems. But changing an RPC is not automatically a way to "stop MEV." Before using a new RPC service, check which network it supports, how transactions are handled, and what privacy or protection features it actually provides. --- ## Practical Ways to Reduce MEV Risk There is no single setting that makes DeFi completely safe, but a few basic habits can reduce unnecessary risk: ### Check Liquidity Before Trading For an unfamiliar token, check the liquidity of the pool before placing a large trade. A small pool can react sharply to a relatively modest transaction. ### Look at Price Impact If the DEX shows a surprisingly high price impact, take a closer look. The problem may be the size of the trade compared with the available liquidity, rather than an MEV attack. ### Do Not Blindly Use Extremely Low Slippage Lower is not automatically safer. A very tight limit can cause a legitimate trade to fail, while a very high limit can leave too much room for an unfavorable execution price. The useful setting depends on the market and the trade. ### Use Swap Protection When Appropriate If the DEX or wallet provides private transaction routing or swap protection, check what it covers before using it. For trades where front-running or sandwich attacks are a concern, private routing can reduce exposure to some of these strategies. ### Read the Swap Screen Before Signing This is easy to ignore when the market is moving fast. Before clicking **Confirm**, check the expected output, minimum output, price impact, slippage, and network fee. If one of those numbers suddenly looks strange, stop. There is no advantage to signing a transaction just because the market is moving quickly. --- ## What About Transaction Deadlines? A **transaction deadline** controls how long a swap can remain valid before it expires. This can be useful when market conditions change quickly. A transaction that remains pending for a long time may eventually be executed under conditions that are very different from the ones that existed when it was submitted. Uniswap's interface currently uses a default deadline of 30 minutes, although the exact behavior can vary by product and transaction. A deadline should not be confused with MEV protection; it simply gives the transaction a time limit. --- ## MEV Is Not Automatically a Scam It is worth keeping the bigger picture in mind. **MEV** is a broad category, not the name of one specific attack. Some MEV comes from arbitrage and other activities that are common in financial markets, and the wider ecosystem can include searchers, validators, block builders, traders, and other infrastructure providers. The more specific concern for an ordinary DEX trader is whether the visibility or ordering of a transaction can be used to make that trade worse. In some situations, yes. That is why sandwich attacks are worth understanding even if the technical details of blockchain infrastructure are not part of your everyday trading routine. --- ## The Bottom Line A DEX swap may look like a simple click on a wallet screen, but behind that click is a much larger system: smart contracts, liquidity pools, pending transactions, automated searchers, block builders, and constantly changing prices. You do not need to become an Ethereum developer to trade responsibly. Knowing what MEV, sandwich attacks, slippage, price impact, liquidity, mempools, private transaction routing, and RPCs actually mean gives you a better chance of spotting a bad trade before signing it. Before a large or unfamiliar swap, check the liquidity and price impact, do not assume that the lowest possible slippage is always the best choice, and review the expected and minimum output before confirming. When private swap protection is available, understand what it does rather than treating it as a guarantee. DeFi can still feel a little like the Wild West, but that is not a reason to stay away—it is a reason to know what is happening on the other side of the **Swap** button.