Block builder

Since the Merge, most Ethereum blocks are not put together by the validator that proposes them. Builders gather transactions from the public mempool, private order flow from wallets and apps, and bundles from MEV searchers, then calculate the ordering that pays the most. They submit the result with a bid to relays, and the proposer picks the highest offer. The difference between what the block earns and what the builder pays is the builder's margin.

Building is a competitive, data-heavy business. Builders that receive exclusive order flow, for example from popular wallets, trading bots or their own trading desks, can outbid rivals, so a handful of builders produce the large majority of Ethereum blocks. Their market share is tracked closely because it shows who effectively decides transaction order.

That concentration creates risks. A dominant builder can exclude transactions, for example to comply with sanctions, delaying them until another builder wins a slot. Builders affiliated with trading firms raise conflict-of-interest questions, and the system still relies on trusted relays until enshrined proposer-builder separation arrives.