MEV, defined
When you send a swap, it sits in a public mempool before a block producer includes it. Whoever assembles that block decides the order of the transactions inside it. They can place their own transactions around yours, buying just before you and selling just after, to profit from the price move your trade causes. That captured value is MEV.
It is not a bug in your swap and not something the venue did wrong. It is a property of how public blockchains order transactions. Anyone who can influence ordering, from block producers to specialized searchers running bots, can compete for it. The larger and more predictable your trade, the more value there is to extract.
Why transaction order is worth money
A blockchain processes transactions in a sequence, and on an automated market maker the sequence changes the price each trade gets. If a bot knows your buy is coming and that it will push a pool's price up, it can buy first at the lower price, then sell into the higher price your order creates. The profit comes entirely from sitting in the right position in the queue.
Because pending transactions are visible before they settle, that position is contested. Searchers watch the mempool, simulate the effect of pending trades, and bid for favorable placement. This competition is the engine behind most MEV that touches an ordinary swap.
How MEV shows up in a swap
The forms that reach everyday swappers are a small subset of what MEV covers, but they are the ones worth recognizing.
- Large orders on thin liquidity are the most exposed, because they move the price the most.
- The cost usually hides inside what looks like normal slippage on the fill.
- Deep, stable pairs at small size leave little for a searcher to take.
- Predictable, publicly broadcast trades are easier to target than private ones.
Sandwich attacks
The most common form for swappers. A bot buys the same asset right before your trade, lets your buy move the price, then sells immediately after. You still get filled, but at a worse rate than the quote implied, and the difference on your receipt looks like ordinary slippage.
Front-running
A searcher spots a profitable pending transaction, copies it, and pays to be ordered ahead of it. This is less about your swap specifically and more about racing you to an opportunity, but it can still change the price you get.
What reduces exposure
You cannot switch MEV off, but you can shrink the surface. The two levers that matter most are how tightly you cap slippage and how your transaction reaches the block.
- A tighter slippage tolerance caps how far a sandwich can move your fill before the trade reverts.
- Private routing sends the transaction to block producers without broadcasting it to the public mempool, so bots cannot see it in time to sandwich it.
- Splitting a very large order, or trading when liquidity is deeper, lowers the price move a searcher can exploit.
- Comparing routes by net received surfaces the path that keeps the most value with you after these effects.
How much it matters
For most everyday swaps on liquid pairs, MEV is a rounding error next to network fees and ordinary price movement. It becomes real on large trades, on thin pairs, and on predictable, publicly visible orders where the price move is worth a searcher's effort. No protection removes every form of extraction, and any approach that sounds absolute is overpromising. The realistic goal is to make your trade an unattractive target rather than an easy one.
This is where an aggregator helps. By comparing routes and ranking them by what actually lands in your wallet, it favors paths that leak less value to reordering. Multiswap compares routes across engines and networks and ranks by net received, which is exactly the number a searcher is trying to erode.