Private Mempool vs Public on Ethereum Flashbots and MEV Protection

05 September 2026

The public mempool is Ethereum's waiting room for unconfirmed transactions. When you submit a transaction, it sits there until a miner or validator includes it in a block. Everyone can see it. Bots watch this room constantly.

MEV (maximal extractable value) bots scan for opportunities. They spot your trade and front-run it, then reverse the trade after you move the price. That sandwich attack inflates your effective cost. You pay more than the slippage you set. The difference goes to the bot.

Private mempools bypass this waiting room. Your transaction goes directly to block builders who agree not to share it until inclusion. The transaction appears only when it is already in a block. Bots cannot see it. They cannot front-run what they cannot read.

Flashbots Protect and MEV Blocker are the two main options for sending transactions through private mempools. Flashbots Protect routes your transaction to a network of searchers and builders; it also checks whether your transaction would revert before sending it. MEV Blocker is a different relay operated by the CoW Protocol team. Both offer similar protection against front-running and sandwich attacks.

Many users believe private mempools guarantee lower gas costs. That is not true. The misconception comes from a real effect: private mempools protect you from sandwich attacks that inflate your effective cost. But the base fee and priority fee are entirely separate. You still pay the Ethereum base fee calculated by EIP-1559, and you still pay a priority fee to incentivize block builders. Private mempools do not reduce those.

They do protect against the hidden cost of MEV. A sandwich attack on a simple swap can cost you 0.5-2% extra on top of fees, and private mempools eliminate that. But the explicit gas you see in MetaMask is the same either way.

To use a private mempool in MetaMask, change your RPC endpoint. Open MetaMask and click the network dropdown, then click "Add network" or edit an existing network. For Flashbots Protect, use the following RPC URL: https://rpc.flashbots.net. For MEV Blocker, use: https://rpc.mevblocker.io. Both are free to use. Set the chain ID to 1 (Ethereum mainnet) and the currency symbol to ETH, then save and switch to that network.

Your transaction now goes through the private relay. It is still an Ethereum transaction. It still pays gas. It still goes in an Ethereum block. The only difference is the order of information flow.

One trade-off: private mempools add latency. Your transaction might take slightly longer to confirm because the relay must process it and forward it to builders, whereas public mempool transactions can be picked up instantly by miners. For most users, this delay is negligible.

Another trade-off: centralization risk. Most private mempool transactions go through a small number of relays and builders; Flashbots alone processes a significant share of Ethereum blocks. If one relay fails or censors transactions, your transaction may not go through. The public mempool is less efficient but more decentralized.

Neither option is perfect for every situation. If you are doing a simple transfer to yourself, the public mempool is fine. If you are swapping large amounts, a private mempool likely saves you money by preventing MEV. If you are interacting with a contract that has unusual gas requirements, test with the public mempool first.

The key takeaway: private mempools protect against front-running, not against gas costs. They make your transaction invisible until it is too late for bots to act. They do not lower the base fee or priority fee. Configure your RPC in MetaMask, understand the trade-offs, and decide based on your transaction type.

Back to gas fees