Visible before it lands
A transaction you sign on Ethereum waits in the mempool until it is included in a block. Transactions in the public mempool are visible to anyone, so what you intend to buy, and how much, is known to other participants before it is confirmed.
This guide shows with a hypothetical swap what public mempool exposure can cost, and explains what sending transactions through private routes instead reduces. It also covers the new trust assumptions and inclusion delays private routes create, and what to check before using one.
What happens in the public mempool
As Ethereum.org's page on transactions explains, signed transactions spread across the network's nodes, and block producers choose which to include. Ethereum.org's page on MEV explains that those who can order transactions, along with searchers, can watch pending transactions and insert their own before or after them for profit.
The best-known case is the sandwich. When a large swap appears in the mempool, someone buys the same token just before it to push the price up, lets the user's swap fill at the higher price, then sells right after. The user tends to be filled at the worst price their minimum output allows.
A hypothetical swap comparison
The table compares a hypothetical swap of 10 ETH for another token sent through three routes. It assumes the minimum output is set 1% below the expected amount.
| Route | Who can see it before inclusion | Sandwich exposure | Hypothetical result | New assumption |
|---|---|---|---|---|
| Public mempool | Anyone | High | Receives about 0.8% less than expected | None |
| Private route to several builders | Route operator and receiving builders | Low | Fills close to the expected amount | Operator and builders do not misuse the information |
| Private route to one builder | Route operator and that builder | Low | Similar fill, but inclusion may take longer | Wait until that builder produces a block |
On a 10 ETH swap, 0.8% is 0.08 ETH. Private routes can reduce this exposure, but they hand the transaction to the route operator.
How private routes work
Flashbots Protect's documentation says the service sends transactions to a private mempool rather than the public one, hiding them from frontrunning and sandwich bots, and refunds part of gas fees and MEV. Users often access such services by changing their wallet's RPC endpoint.
A privately sent transaction is passed by the route operator directly to block builders and becomes public only once included in a block. Until then, the parties who can see it shrink to the operator and the builders that received it.
New trust and delay
Private routes do not remove exposure; they narrow who can see a transaction. They introduce the assumption that the operator and builders will not misuse what they receive, and if the operator stops or rejects certain transactions, inclusion can be delayed.
If the receiving builders do not win the right to build a block, inclusion waits. Sending to a single builder, as in the table's third row, means waiting until that builder produces a block, which can hurt urgent transactions. When prices move fast, delay is a cost of its own.
There is also a privacy angle beyond MEV. A private route operator sees your IP address and every transaction you send through it, so its data policy matters in much the same way an RPC provider's does.
Checks before using one
Before using a private transaction route, work through these items.
- Confirm the RPC address you are adding matches the one in the service's official documentation.
- Check which builders receive your transactions and whether you can choose them.
- Find out how long before an unincluded transaction is dropped, and whether it is ever sent to the public mempool.
- Read the refund policy and how the service handles your IP address and account information.
- Keep a sensible minimum output on swaps even when using a private route.
Is it needed for every transaction?
Simple transfers, where reordering yields no profit, carry little MEV risk even in the public mempool. Private routes matter most for transactions where order and price shape the outcome, such as large swaps on decentralized exchanges, liquidations, and purchases of newly launched tokens.
MEV is not solved by individual users' choices alone. The Ethereum community is discussing several changes to block production and ordering rules, so the role and risks of private routes may change over time.
Limits of the comparison
The loss rate and route comparison here are hypothetical. Real MEV varies widely with trade size, liquidity, and market conditions, and private route services change their policies often.
When using such a service, check Ethereum.org and its current documentation with their dates. This guide does not recommend using any service or trading any token.