Proposer-Builder Separation (PBS) is the architectural concept — and increasingly the practice — of dividing Ethereum’s block production process into two distinct roles: builders who assemble and optimize block contents (ordering transactions, capturing MEV, filling blocks with optimal transaction sets), and proposers (validators) who select the most profitable block from a competitive builder market and add it to the chain without building it themselves. PBS emerged from Ethereum research as the principled response to MEV centralization: if only the most sophisticated actors can profitably maximize block value, and validators build their own blocks, then validators are pushed toward centralized, sophisticated operators. PBS lets validators remain simple and decentralized while professional builders compete in an auction.
How It Works
The following sections cover this in detail.
The Problem PBS Solves
In Ethereum’s original design, validators both propose the block and choose its contents. This means a validator who can extract more MEV (by reordering transactions, including sandwich trades, etc.) earns more than one who can’t. This creates pressure to:
- Centralize validators under sophisticated MEV-capture operations
- Outsource block building to MEV specialists, creating off-chain power concentration
PBS formalizes this separation with proper incentive structures.
Current Implementation: MEV-Boost (Out-of-Protocol PBS)
Ethereum doesn’t yet have PBS natively in the consensus protocol (as of 2025). The current implementation is MEV-boost — a middleware solution developed by Flashbots:
- Builders construct full blocks and submit them to relays (trusted intermediaries) with a bid (how much ETH they’ll pay the proposer)
- Relays verify that builder blocks are valid and relay bids to validators
- Proposers (validators) run MEV-boost, which queries connected relays for bids and selects the highest-paying block
- The proposer commits to the highest bid by signing the block header; the relay reveals the full block contents only after receiving the proposer’s commitment
- The proposer publishes the full block to the network
This commitment-reveal scheme prevents the proposer from stealing the block contents (seeing the MEV, taking it, and proposing a different block).
Relay Trust Problem
MEV-boost relays are trusted third parties — a required concession in the current out-of-protocol implementation. Relays can:
- Censor transactions (including OFAC compliance censorship, which became significant post-Tornado Cash sanctions)
- Be a centralization point (a relay going down disrupts all connected validators)
- Potentially collude with builders or proposers
This is why ePBS (enshrined PBS) — where PBS is built into Ethereum’s consensus protocol itself, eliminating relay trust — is a major Ethereum roadmap goal. ePBS was designed alongside the development of “attester-proposer separation” and related research.
Builder Market Landscape
The builder market is currently dominated by a small number of entities:
| Builder | Market Share (approximate) | Notes |
|---|---|---|
| Titan Builder | ~25-35% | Consistently dominant |
| beaverbuild | ~20-30% | Flashbots affiliated |
| rsync-builder | ~10-20% | Large order flow aggregator |
| Flashbots builder | ~5-15% | Original MEV-boost pioneer |
Builder concentration is a significant concern — if 2-3 builders control 80%+ of blocks, the Ethereum block space market is effectively oligopolistic, even though validators remain decentralized.
Key Terms in PBS Ecosystem
- Proposer: Validator randomly selected to propose the next block in a slot
- Builder: Specialized off-chain entity constructing optimal block content and bidding for proposer inclusion
- Relay: Trusted intermediary that verifies builder blocks and passes bids to proposers; reveals block body only after proposer commitment
- MEV-boost: The open-source Flashbots middleware that implements out-of-protocol PBS
- ePBS: Enshrined PBS — the planned native consensus-layer implementation eliminating relay trust
- PBS Commit-Reveal: The cryptographic scheme ensuring proposers can’t steal builder MEV
History
- 2021 — Flashbots publishes the original PBS proposal and MEV-boost design, responding to the chaotic “dark forest” of uncoordinated MEV extraction.
- 2021 — MEV-Boost development begins as a practical out-of-protocol implementation for Ethereum’s Proof of Stake transition.
- 2022, September — The Merge — Ethereum transitions to Proof of Stake; MEV-boost immediately deployed by the majority of validators.
- 2022 — OFAC censorship controversy — Post-Tornado Cash sanctions, some relays began censoring OFAC-sanctioned addresses; within 2 weeks of The Merge, >50% of blocks were being built through censoring relays. This accelerated ePBS urgency.
- 2023 — Inclusion lists proposed as a mechanism to ensure proposers can force certain transactions into blocks even with PBS.
- 2023–2024 — ePBS research intensifies; Justin Drake and others publish ePBS designs; becomes a target for inclusion in Ethereum’s Fusaka/Glamsterdam upgrade roadmap.
- 2025 — Builder concentration concerns continue; multiple Ethereum researchers publish analyses of block building market structure.
Common Misconceptions
“PBS eliminates MEV.”
PBS does not eliminate MEV — it structures who captures it and how it is distributed. MEV still exists in the form of ordering transactions and arbitrage. PBS routes MEV value to validators (via builder bids) rather than allowing validators to extract it adversarially. MEV-burn proposals aim to return some portion to the network, but that’s a separate design question.
“MEV-boost is the same as enshrined PBS.”
MEV-boost is an out-of-protocol stopgap requiring relay trust. Enshrined PBS eliminates the relay by putting commitment-reveal mechanics directly into the consensus protocol — a more trust-minimized design. They achieve similar separation goals through different mechanisms.
“All validators use MEV-boost.”
As of 2025, ~85-90% of Ethereum validators use MEV-boost. Validators can opt out and build blocks locally (called “vanilla blocks”), accepting lower average rewards in exchange for not relying on relay infrastructure.
Criticisms
- Builder centralization — PBS was designed to keep validators decentralized, but it shifted centralization pressure to the builder market. A handful of sophisticated builders capturing the majority of blocks represents a new centralization vector at a different layer.
- Relay censorship — Current MEV-boost relays have censored transactions (OFAC compliance), demonstrating that PBS’s relay layer is a censorship point. ePBS is designed to address this but remains unimplemented.
- Value extraction from users — PBS formalizes and optimizes MEV capture rather than reducing it. Users who have their transactions reordered for MEV purposes are still harmed; PBS just routes that value to validators instead of informal operators.
- Proposer timing games — Under PBS, proposers who delay revealing their block selection can extend the slot window to see competing attestations, creating “late block” strategies that destabilize consensus timing.
Social Media Sentiment
PBS and MEV-boost were major Ethereum Twitter/X topics surrounding The Merge (September 2022). The OFAC censorship discussion generated intense debate about Ethereum’s censorship resistance — tracking dashboards like mevwatch.info showed real-time block censorship percentages, fueling daily discourse. r/ethereum consistently treats PBS as essential but not sufficient infrastructure: appreciated as decentralized validator protection, criticized as enabling builder oligopoly. Ethereum researchers (Buterin, Justin Drake, Barnabé Monnot) regularly publish PBS design updates that circulate heavily in the technical community.
Last updated: 2026-04
Related Terms
Sources
- Buterin, V. (2021). Two-Slot PBS. Ethereum Research (ethresear.ch).
- Flashbots (2021). MEV-Boost: Proposer-Builder Separation for Ethereum. github.com/flashbots/mev-boost.
- Monnot, B. (2022). “Ethereum’s Proposer-Builder Separation and MEV Distribution.” EF Research Blog.
- Schwarz-Schilling, C. et al. (2023). Notes on Proposer-Builder Separation. Ethereum Research.