DeFi · 58 / 86 · 2 min
MEV
MEV is value taken by whoever can reorder, insert, or censor transactions. Your swap can be pushed behind someone else's, so you get a worse price.
MEV is value taken by whoever can reorder, insert, or censor. Your swap can be pushed behind someone else's.
A worked case
A person sends a large swap with loose slippage. A searcher buys first and sells into the swap. The person receives less. The block was valid. The order of the transactions was the product.
In front of you
You submit a trade. A searcher sees it, buys first, lets your trade push the price, and sells. You paid the slippage. They took the difference. This is often called a sandwich. You do not need the method. You need to know the price can be worsened on purpose.
What you can actually do
Keep slippage tight so a bad fill fails instead of draining you. Avoid announcing a huge trade in a public pool with a loose limit.
The chain's public waiting room is not a private conversation.
Your swap was not alone in the block
Maximal extractable value is the money available to whoever orders transactions in a block. A large swap with loose slippage can be seen in the public waiting room. A searcher buys first, your swap pays the worse price, and they sell. The block is valid. The Ethereum community spent the years after the Merge building private relays and order-flow auctions so that this would at least be competed over, rather than stolen in the dark. Competed over is not the same as gone.
You feel it as a worse fill than the quote, or as a failed transaction that still cost gas because someone else moved the pool. Tight slippage refuses the bad fill and sometimes refuses the trade. That is the tradeoff. Loose slippage gets you the tokens and donates the difference.
The builder and the validator who order the block are who get paid. Your signature allowed the range. If you do not set the range, you set it to whatever the wallet suggested, which was chosen so the swap would not fail.
Read it yourself
These links are the record. They are not a recommendation, and they are not instructions. A news story or a court paper can still be wrong about a detail. The check does not change because a famous name is in the story.
Apply the check
Open a question. The line is about this topic. It is not a verdict that anything is safe.
The searcher who reordered the block.
The builder or validator who orders transactions.
The worse price, inside the slippage you allowed.
Check yourself
Is the quoted swap price safe because you clicked quickly?
No. Someone who sees the public transaction can still get in front of it.
After this you can set a tight minimum on a swap so a worsened price fails instead of filling.