Relay
In the MEV-Boost architecture, a relay is a trusted intermediary that receives blocks from builders, validates them, and forwards them to proposers (validators). The relay's critical function is data withholding: it reveals the block's bid value to the proposer but withholds the block contents until the proposer commits to selecting that block. This prevents proposers from unbundling an MEV-rich block and extracting the value themselves, which would undermine the builder market. Relays also validate block construction (ensuring correct state transitions and respecting gas limits) and maintain builder reputation. The relay is a centralization point and trust assumption in the current PBS design — a malicious relay could censor transactions, discriminate among builders, or extract MEV. The Ethereum protocol's eventual enshrined PBS upgrade aims to move relay functionality into the protocol itself, eliminating the need for trusted third-party relays.