Block Builder
A block builder is a specialized entity in the PBS architecture that constructs execution payloads — ordering transactions within a block to maximize the value that can be extracted. Builders operate algorithms that simulate thousands of transaction orderings, identifying the arrangement that yields the highest total value (including MEV opportunities) while respecting constraints like gas limits and transaction dependencies. The builder submits their proposed block to a relay along with a bid for the proposer; the proposer (validator) selects the block with the highest bid without seeing the full block contents ahead of time. The builder market is highly competitive and concentrated — a handful of specialized builders (Beaverbuild, Titan, rsync) consistently produce the majority of blocks because they have developed the most sophisticated extraction algorithms and maintain low-latency infrastructure. Builder revenue comes from MEV extraction minus the bid paid to the proposer minus execution costs.