proposer-builder separation

Proposer-builder separation (PBS) splits one job — making a block — into two distinct roles: the builder, who decides what goes inside the block (which transactions, in which order), and the proposer, who merely commits the finished block to the chain. The motivation is that profitably ordering transactions to capture MEV has become a specialized, capital-intensive activity. If every block proposer also had to be an expert MEV extractor, only a few sophisticated operators could compete, and proposing — a role meant to be open to anyone with a small stake — would centralize.

PBS breaks that link. Builders compete in an open market to construct the most valuable block and bid for the privilege of having it proposed; the proposer simply chooses the highest bid and signs it, needing no MEV expertise at all. A home staker with 32 ETH can thus earn essentially the same block rewards as a giant staking operation, because both just accept the best bid from the builder market. The economic value of ordering flows to builders through competition, while the security-critical act of proposing stays maximally decentralized.

Crucially, the proposer commits to a block before seeing its contents — it signs a block header, not the transactions — so a builder's profitable arrangement cannot be copied and stolen by the proposer. Today this is implemented out-of-protocol via MEV-Boost, where relays broker the commit-reveal exchange and stand in as trusted parties. Enshrined PBS is the research goal of building this separation directly into the Ethereum protocol, so the fair exchange is guaranteed by consensus rules rather than by trusting a relay.

PBS is a deliberate trade: it concentrates the power to order transactions (and therefore to censor) into a smaller set of builders, in exchange for keeping the much larger set of proposers decentralized and the validator role accessible. The danger is builder centralization and censorship, which is why PBS designs are paired with mechanisms like inclusion lists — letting the decentralized proposer force certain pending transactions to be included regardless of what the builder preferred — to preserve censorship resistance even though block contents are chosen by specialists.