block relay

In the MEV-Boost world, a relay is a trusted intermediary that stands between block builders and the validators who propose blocks. Builders do not want to hand their carefully crafted, profitable block directly to a proposer who might copy its contents and steal the value; proposers do not want to commit to publishing a block they cannot see. The relay solves this standoff by sitting in the middle: it receives full blocks from builders, validates them, shows proposers only the headers and bids, and reveals each winning block's body at the right moment.

Mechanically, a relay performs several jobs. It accepts complete block submissions from many builders and checks each one is valid and that the builder can actually pay the promised bid (guarding against proposers being tricked into signing for a bid that will not materialize). It ensures the block's data is available — it must hold the full body so it can publish it. It forwards only the highest-bidding header to the proposer via MEV-Boost, and after the proposer signs that header, it broadcasts the corresponding full block to the network. It also enforces that a proposer cannot get two different bodies for one signed header.

Because the relay sees full blocks before anyone else and gatekeeps what reaches proposers, it concentrates trust and power. A faulty or malicious relay can withhold a block body after a proposer has already committed to its header, causing that validator to miss its slot entirely and lose rewards. A relay can also choose to filter certain transactions — for example, to comply with sanctions lists — which is how block-level censorship enters an otherwise permissionless system. There are only a handful of widely used relays, so their policies have outsized influence over what gets included on Ethereum.

The honest framing is that relays are a pragmatic, temporary scaffolding, not the intended end state. They exist because out-of-protocol proposer-builder separation needs someone to broker the commit-reveal exchange and enforce data availability and payment. Ethereum's research roadmap aims to dissolve this trusted role through enshrined PBS and proposer-side inclusion lists, which would let the protocol itself guarantee fair exchange and censorship resistance, removing the relay as a single point of trust and potential failure.