PBS and MEV-Boost

The Problem PBS Solves

Before Ethereum's Merge in 2022, MEV extraction (the profit from ordering transactions strategically — sandwich attacks, arbitrage, liquidations) was captured mostly by validators who had specialised infrastructure to build MEV-optimised blocks themselves. This created two problems: (1) it advantaged validators with sophisticated in-house MEV expertise over solo stakers, and (2) it required every validator to be a competent block builder, which was operationally demanding. Proposer-Builder Separation (PBS) is the design pattern that splits these two roles: block builders specialise in constructing MEV-optimised blocks, and block proposers (validators) simply select the most profitable pre-built block via a bidding market. MEV-Boost is the current out-of-protocol implementation of this pattern.

How MEV-Boost Works

When a validator is about to propose a block, they query one or more relays. Each relay collects blocks from various builders and passes on the highest-bidding block to the validator. The validator signs and proposes that block without knowing the exact contents (only that it will pay them the bid amount). The relay ensures the builder actually delivers the block after the validator commits. This flow lets solo validators earn MEV revenue nearly equal to what sophisticated in-house builders earn — the market handles the specialisation. As of October 2026, roughly 90% of Ethereum blocks are produced through MEV-Boost (an estimate from relayscan.io's 24-hour builder block counts against the 7,200 slots in a day).

The Trust Assumptions

MEV-Boost requires trusting the relay: the relay knows both the block contents and the validator's identity before the block is proposed, giving it the power to censor transactions or leak information. In practice this hasn't been a widespread problem, but it's a real trust surface. Relays are operated by a small number of organisations, and several relays can deliver the same block. Relays delivering payloads in relayscan.io's 24-hour window to 01:58 UTC on 11 October 2026 included Ultra Sound Money, Titan, bloXroute's regulated relay, Aestus, Flashbots, Agnostic and EthGas. Different relays have different filtering policies (which transactions they include, which they don't), which itself has been a subject of ongoing debate.

• PBS splits block production into building (MEV-optimised) and proposing (validator)
• MEV-Boost is the current out-of-protocol PBS implementation
• Roughly 90% of Ethereum blocks went through MEV-Boost as of October 2026 (estimate derived from relayscan.io 24-hour block counts, 11 October 2026)
• Trust surface: validators trust relays not to censor or leak block contents