MEV-Boost Architecture: PBS, Relays, and Protection

Frequently Asked Questions

01.What is proposer-builder separation in MEV-Boost?

Proposer-builder separation (PBS) splits Ethereum block production into two roles: builders assemble the most valuable block from transactions and MEV bundles, while the proposer, a validator, selects and signs the highest-bidding block header. MEV-Boost is the software that implements this off-protocol, with a relay sitting between builders and proposers. The split means validators capture MEV revenue without running sophisticated block-building infrastructure, and it is why MEV-Boost adoption became near-universal among validators.

02.What does a MEV-Boost relay do?

A MEV-Boost relay is a trusted mediator between block builders and validators. It receives full blocks from builders, validates and simulates them, then forwards only the block header and bid value to the proposer while holding the full payload in escrow. After the proposer signs the header committing to that block, the relay releases the payload. This commit-before-reveal flow is what prevents a validator from stealing the builder's MEV by copying its transactions.

03.What is an optimistic relay?

An optimistic relay reduces auction latency by skipping synchronous block simulation, marking a builder's bid active immediately and simulating asynchronously. Builders post collateral that refunds proposers if an invalid block is delivered, and relays retain the ability to intervene against dishonest builders. Optimistic relaying removed roughly 150 milliseconds of simulation latency in early deployments, which improved builder competitiveness and proposer fees at the cost of a new collateral-based trust assumption.