PBS, MEV-Boost, Builder Landscape

What you'll learn

• Why PBS Exists
• 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.