A block builder is a highly specialized entity in the Ethereum post-Merge MEV (Maximal Extractable Value) supply chain that receives transaction bundles from MEV searchers, mempool transactions from users and relayers, and private order flow from DeFi protocols and wallets, and then constructs the most economically valuable possible block by solving an optimization problem — determining the exact transaction ordering, inclusion set, and sequencing that maximizes the total extractable value — before submitting this sealed block header to validators via MEV-Boost, accompanied by a bid (in ETH) that the validator earns if they choose to include the builder’s block rather than building locally. Block builders emerged as a distinct role following Proposer-Builder Separation (PBS) — the practical implementation of which is MEV-Boost, a middleware developed by Flashbots — which separated the roles of block proposer (the validator who must be randomly selected by the network) from block builder (an entity that can specialize in maximizing block value through optimal transaction ordering).
Why Block Builders Exist
Before PBS/MEV-Boost (pre-Merge and early post-Merge), validators built their own blocks — selecting transactions from the public mempool ordered by gas price. This created problems:
- Unsophisticated validators lost MEV: Validators running simple nodes couldn’t capture the MEV available in optimal transaction ordering
- Priority gas auctions: Searchers competed for inclusion by bidding up gas, clogging the network
- Centralization risk: Only validators with sophisticated MEV software could capture full block value, creating incentives to centralize staking
MEV-Boost solved this by creating a marketplace: validators simply choose the highest bid from competitive builders, outsourcing the block construction work.
How Block Building Works
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INPUT:
├── Private order flow (from wallets, DEX aggregators, protocols)
├── Searcher bundles (from Flashbots relay, other relays)
├── Public mempool transactions (from Ethereum p2p network)
└── Sandwich/backrun opportunities identified by builder’s own bots
BUILDER’S OPTIMIZATION PROBLEM:
├── Which transactions to include (subject to gas limit)
├── What order to sequence them (maximize MEV extraction)
├── How to combine searcher bundles without conflicts
└── How much to bid to win against competing builders
OUTPUT:
└── Sealed block header + bid submitted to relay
└── Validator sees only: bid amount + block header hash
└── Validator cannot see contents (prevents stealing MEV)
└── Validator signs header if bid is highest
└── Builder releases full block; included in chain
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The Builder Market
Post-Merge, a small number of builders have dominated Ethereum block production:
| Builder | Typical Market Share | Notes |
|---|---|---|
| beaverbuild | 25–40% | Largest consistently; also runs a DEX |
| Flashbots (builder0x69) | 10–20% | Original Flashbots builder |
| rsync-builder | 10–15% | High private order flow share |
| Titan Builder | 5–15% | Aggressive in wallet partnerships |
| f1b.io | 5–10% | Focused on specific MEV types |
Builder concentration is a significant concern: if 2–3 builders control 70%+ of blocks, they have the ability to censor specific transactions, charge “builder’s rent” via MEV extraction, or collude with validators.
Builder Revenue Sources
Builders extract and distribute value from multiple sources:
1. MEV from Searcher Bundles
2. Proprietary MEV
3. Private Order Flow
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User swaps on a MEV-protected wallet →
Wallet routes to builder X privately →
Builder X can still MEV the user (softly) but protects from sandwich attacks →
Builder X pays back some MEV value as “cashback” to the user →
Builder X gains exclusive access to valuable order flow
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Exclusive private order flow creates a moat for dominant builders — new entrants cannot compete without comparable order flow.
Builder vs. Relay vs. Validator
| Role | Function | Ethereum Entity Type |
|---|---|---|
| Searcher | Finds MEV opportunities; submits bundles | Software bots |
| Builder | Constructs optimal block; submits to relay | Specialized firms |
| Relay | Escrow between builder and validator; verifies bids | Trusted intermediary (Flashbots, BloXroute, etc.) |
| Validator | Proposes the chosen block; earns builder bid | Stakers with 32+ ETH |
The relay is a critical trust assumption: it must honestly communicate the block value to validators without leaking the block contents (which would allow validators to steal MEV). Relay centralization (most validators use Flashbots relay) is another systemic concern.
Censorship and Builder Risk
Because builders control transaction inclusion, they have the ability to censor specific transactions:
- OFAC-compliant builders in 2022–2023 excluded transactions from sanctioned Tornado Cash addresses
- At peak, 70%+ of Ethereum blocks were OFAC-compliant, meaning sanctioned transactions faced indefinite delays
- This prompted debate about whether PBS undermines Ethereum’s censorship resistance
A validator building locally (not using MEV-Boost) cannot censor transactions — but earns substantially less. This creates a financial incentive that pushes toward compliant builder usage.
History
- August 2021: Flashbots launches MEV-Geth (miner-side MEV for PoW); prototype of builder concept
- September 2022: Ethereum Merge; MEV-Boost launches day-one; validators immediately adopt it
- October 2022: 80%+ of Ethereum blocks built via MEV-Boost within weeks of launch
- 2022–2023: OFAC compliance controversy; Ethereum community debates censorship resistance
- 2023: Builder market consolidates to ~5 dominant entities; private order flow wars begin
- 2024: “Enshrined PBS” (ePBS) — a native Ethereum protocol change to formalize builder separation — enters Ethereum research roadmap
- 2025: Builder concentration remains a structural concern; L2 sequencer centralization raises analogous issues on rollups