Proposer-Builder Separation (PBS) is one of the most consequential architectural changes in Ethereum’s post-Merge evolution. Before PBS, validators were expected to build their own blocks — selecting transactions from the mempool and ordering them. This created a massive disadvantage: validators without sophisticated MEV extraction software earned far less than those who could profit from transaction ordering (arbitrage, liquidations, sandwich attacks). PBS splits these duties: builders compete to construct the most profitable block possible (using complex ordering strategies), and validators (proposers) simply select the highest-bid block from a marketplace and sign it — without needing to know why it’s profitable or reorder anything. MEV-Boost (by Flashbots) implements an out-of-protocol version of PBS that is currently live on Ethereum, with in-protocol PBS (ePBS) planned for future Ethereum upgrades.
How PBS Works
Current Implementation: MEV-Boost (out-of-protocol PBS)
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Searchers → Builders → Relay → MEV-Boost → Validator (Proposer)
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- Searchers: Find profitable MEV opportunities (arb, liquidations, sandwiches), submit bundles to builders
- Builders: Construct full blocks combining mempool txs + searcher bundles, maximize reported value
- Relays: Trusted intermediaries that verify blocks and forward bids to validators (Flashbots Relay, BloXroute, etc.)
- MEV-Boost (on validator): Receives bids from multiple relays, selects highest-paying block header, signs it
- Validator: Proposes the winning builder’s block — earns the bid value as MEV boost on top of consensus rewards
Key Concepts
| Component | Role |
|---|---|
| Proposer (validator) | Signs blocks, doesn’t see tx contents until committed |
| Builder | Constructs block, pays bid to proposer |
| Relay | Trusted intermediary; ensures builders can’t cheat proposers |
| Block auction | Proposer selects highest-bid header without seeing full block |
| Commitment scheme | Builder commits to block before proposer reveals it |
PBS Impact on Ethereum
- Validator rewards: ~90% of ETH validators use MEV-Boost; adds 20-100%+ to validator rewards vs. vanilla blocks
- Centralization risk: Block building is now dominated by a handful of sophisticated entities (Titan, beaverbuild, Flashbots)
- Censorship risk: Relays can filter transactions (OFAC compliance); censorship incidents observed post-Tornado Cash sanctions
- Democratization outcome: PBS successfully democratized MEV revenue for validators (any staker earns MEV), but centralized block construction
ePBS (In-Protocol PBS)
The current MEV-Boost implementation relies on relays as trusted intermediaries — a semi-centralized component. Ethereum’s long-term roadmap includes enshrined PBS (ePBS) that builds the builder/proposer separation directly into the consensus protocol, eliminating relay trust. EIP-7732 is the leading ePBS proposal as of 2024.
Social Media Sentiment
PBS is discussed extensively in Ethereum developer and validator communities. Early PBS critics warned of centralization in block building (confirmed — 2-3 builders dominate). MEV researchers generally view PBS as a necessary intermediate step toward more decentralized block production. Concerns about censorship resistance (relay filtering of OFAC-sanctioned addresses) remain active governance discussions in the Ethereum community.
Last updated: 2026-04
Sources
- Ethereum.org — Proposer-Builder Separation — Ethereum Foundation reference for PBS architecture and motivation.
- Flashbots — MEV-Boost — the out-of-protocol PBS implementation running on Ethereum.
- EIP-3675 — Casper FFG (Merge) — the underlying Merge upgrade that enabled the post-Merge validator model PBS is built upon.
Related Terms
Sources
- “Proposer-Builder Separation: Ethereum’s MEV Architecture” — Buterin, Chitra, et al. / Ethereum Research (2021-2022). The original Ethereum research posts and EIPs proposing PBS — describing the architecture, motivations, and implementation path.
- “MEV-Boost: Flashbots’ Out-of-Protocol PBS” — Flashbots (2022). Technical documentation of MEV-Boost — the middleware software running on >90% of Ethereum validators that implements the PBS auction externally to the protocol.
- “Builder Centralization in Ethereum: A Threat to Decentralization?” — Barnabé Monnot / EF Robust Incentives Group (2023). Empirical analysis of builder market concentration — measuring the Herfindahl-Hirschman Index (HHI) of the builder market and evaluating implications for Ethereum decentralization.
- “PBS and Censorship Resistance: OFAC Filtering and the Relay Debate” — Ethereum Censorship Dashboard / Rated.network (2022-2023). Analysis of OFAC transaction filtering by PBS relays post-Tornado Cash sanctions — measuring censorship rates and implications for Ethereum’s neutrality.
- “EIP-7732: Enshrined PBS (ePBS) Specification” — Ethereum Core Developers (2024). The leading formal proposal for in-protocol PBS — eliminating the trusted relay dependency by implementing builder/proposer separation natively in the Ethereum consensus protocol.