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PBS, MEV-Boost, Builder Landscape

Proposer-Builder Separation and the MEV-Boost market. Beaverbuild, Titan, Flashbots, rsync. Relays, OFAC compliance, and SUAVE's proposed neutral block-building network.

30 min · expert · part of MEV Searcher Strategies

Why PBS Exists

Proposer-Builder Separation (PBS) is the architectural change that defines post-Merge Ethereum's MEV landscape. The idea is simple: validators (proposers) should not also be the ones building blocks. Block-building is a specialized, latency-sensitive, capital-intensive activity. If every validator had to also be a sophisticated MEV-extractor to remain competitive, the validator set would centralize into a small number of well-resourced firms. PBS lets validators stay validators while a competitive market of dedicated builders handles the block-construction work. The implementation in production is **MEV-Boost** — a piece of middleware that runs alongside the validator's consensus client. MEV-Boost was created by Flashbots and launched September 6, 2022 (the day before the Merge) as a voluntary opt-in for validators. Within weeks roughly 70% of Ethereum validators had opted in; by mid-2025 the share runs around 90-95%, with the remaining 5-10% choosing to build blocks locally for ideological or regulatory reasons. The architecture has three roles. **Searchers** find profitable transaction sequences and bundle them. **Builders** aggregate bundles, public mempool transactions, and private order flow into complete blocks, then bid for those blocks to be proposed. **Relays** sit between builders and validators, validating that the proposed block is well-formed and that the bid is paid to the proposer. The validator (running MEV-Boost) accepts the highest valid bid blindly — they never see the block contents until after committing to propose it, which prevents them from stealing the MEV themselves. This lesson covers the major players in each role, the OFAC compliance debate that fractured the relay landscape in 2022-2023, and SUAVE — Flashbots' proposed decentralized successor to the current builder market.

Also in this lesson

  • The MEV-Boost Flow End to End
  • The Major Builders
  • Relays and the OFAC Compliance Debate
  • SUAVE: Decentralizing the Builder

Key terms

PBS (Proposer-Builder Separation)
Architectural pattern where block-proposing validators outsource block construction to specialized builders. Implemented in production via MEV-Boost as opt-in middleware.
MEV-Boost
Flashbots-built middleware launched September 6, 2022 (day before the Merge). Lets validators receive blocks from builders via relays and propose the highest bid blindly. Adopted by ~90-95% of validators by 2025.
Builder
A specialized firm assembling Ethereum blocks from mempool transactions, searcher bundles, and private order flow. Major builders by 2025-2026: Beaverbuild (~30-40%), Titan (~20-30%), Flashbots (~15-25%), rsync.
Relay
Trust-minimized middleman between builders and validators. Validates blocks, escrows headers until signed, then reveals bodies. Major relays: Flashbots (compliant), Ultra Sound (non-compliant), Aestus, Agnostic, Titan, BloxRoute.
OFAC-compliant relay
A relay that filters out blocks containing transactions involving US-sanctioned addresses (Tornado Cash, etc.). Flashbots, BloxRoute Regulated, Manifold. Created the original 2022 censorship debate.
Censoring slot
A block that was proposed exclusively through OFAC-compliant relays. Tracked by mevwatch.info; share fluctuates 25-50% by 2024-2026 but has not reached 100%, so sanctioned transactions remain includable with delay.
SUAVE (Single Unifying Auction for Value Expression)
Flashbots' proposed decentralized successor to MEV-Boost. Separate chain where encrypted bundles are processed by competing executors inside TEEs. Rigil testnet running 2025-2026; mainnet target 2026-2027.
Inclusion list
Proposed Ethereum-protocol-level change (EIP-7547 / Glamsterdam fork variant) letting proposers force inclusion of specific transactions, providing a censorship-resistance backstop independent of relay behavior.
Vertical integration
When a builder also runs in-house searching operations, creating an information advantage when constructing blocks (knowing internal bids before exposing opportunities). Structural issue in current PBS implementation.

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Educational only — not financial advice.