Definition
A block builder is a network role responsible for constructing candidate blocks from a set of available transactions, typically sourced from the mempool or similar transaction pools. Instead of simply packing transactions in arbitrary order, a block builder prioritizes them according to fees, incentives, and potential MEV opportunities. In systems that separate block proposal from block construction, the block builder focuses on optimizing the economic value of the block, while a validator or sequencer ultimately handles block proposal or block execution. The block builder’s output is a fully ordered block or block template that can be submitted into a competitive blockspace market.
In advanced architectures, block builders operate as distinct entities that specialize in block building as an economic and technical function. They may compete to produce the most valuable block for a given slot or time interval, with the winning block selected through a block proposal mechanism. This separation of roles can change how MEV is captured and distributed, and can alter incentives for validators and other participants. As a result, the block builder has become a central concept in discussions of blockspace market design and the structure of modern blockchain consensus pipelines.
Context and Usage
The term block builder is often used in contrast to a validator that both proposes and executes blocks in more traditional designs. In architectures where block building is outsourced, validators or sequencers may rely on external block builders to supply pre-constructed blocks, focusing their own role on block proposal and consensus participation. This creates a layered structure in which block building, block proposal, and block execution can be handled by different specialized actors.
Within a blockspace market, block builders act as key intermediaries between transaction originators and the entities that finalize blocks on-chain. Their strategies for ordering, including MEV-aware transaction placement, directly influence which transactions are included and in what sequence. As research and practice around block building evolve, the definition of the block builder role continues to be refined, but it consistently denotes the actor that transforms raw transaction flow into economically optimized candidate blocks.