Summary
Exclusive order flow — private transaction routing from wallets, OFA providers, and searchers to specific builders — is the primary driver of block building market concentration. The flywheel dynamic (more flow → higher bids → more validators → more flow) has produced a market where two builders construct ~74% of all blocks. Relay bid adjustment and unsustainable relay economics are secondary structural symptoms.
What Is Exclusive Order Flow?
“Order flow” refers to transactions destined for on-chain execution. “Exclusive” means a transaction (or bundle) is sent only to one or a few builders, rather than broadcast to all.
Sources of exclusive flow:
- OFA providers (Order Flow Auctions): Flashbots Protect, bloXroute BDN, MEV Blocker — user-facing RPCs that auction backrun rights on user transactions to searchers, paying the user a rebate.
- Wallet integrations: wallets route transactions through preferred RPCs for MEV protection or fee rebates.
- CEX-DEX arbitrage: fast traders with exchange API access send time-sensitive arb bundles privately to avoid being sniped.
- Searcher-builder agreements: searchers develop exclusive relationships with builders who offer favorable bundle inclusion policies.
The Flywheel
Exclusive flow → Winning blocks → Higher proposer bids → More validator adoption
↑ |
└──────── More OFA/wallet/searcher partners ←─────────────────┘
- A builder wins more blocks because they have exclusive access to high-value flow.
- They consistently bid higher than competitors.
- Validators using MEV-Boost observe higher earnings from this builder.
- More validators include this builder’s relay.
- The builder’s blocks are selected more often.
- OFAs and searchers see this builder as the most reliable inclusion path → route more flow to them.
This is a winner-takes-most dynamic with strong incumbency advantages.
Quantitative Impact
- Exclusive order flow can account for up to 84% of winning block fees in some blocks.
- The top 2 builders (Titan, BuilderNet) construct ~74% of blocks (2026 data).
- The PBS premium over local builds is ~400% — validators are highly dependent on a healthy, competitive builder market.
Relay Bid Adjustment
A related but distinct problem: relays occasionally adjust builder bids — raising or lowering the value reported to proposers relative to what the builder submitted.
- ~5% of slots are affected
- ~2% of total bid value is adjusted
- Relays have economic incentives to adjust (e.g., taking a fee) and no protocol mechanism prevents it
- Builders have limited recourse; proposers can’t detect it
This is a trust/honesty problem with no current remedy short of ePBS (which removes the relay’s role).
Relay Economics
Relays have no sustainable business model:
- They cannot charge builders without losing volume to competing free relays.
- They cannot charge proposers without losing validators to free relays.
- They bear significant infrastructure costs (simulation, low-latency co-location, global presence).
- Result: relays are subsidized by their parent organizations or operate at a loss.
The “Observation on Ethereum’s Blockspace Market” paper identifies this as a structural gap that makes the relay layer fragile.
Implications for Censorship
When exclusive OFA flow is concentrated with OFAC-compliant builders/relays, it can lead to transaction censorship. After OFAC listed Tornado Cash in August 2022, OFAC-compliant relay share reached ~79%. Community pressure brought this down to ~27% by mid-2023, but the architecture makes it structurally possible for regulation in a single jurisdiction to affect a large fraction of blocks.
Builder Strategies
| Strategy | Description | MEV type |
|---|---|---|
| Exclusive OFA partnerships | Preferred builder for MEV Blocker, CoW, etc. | Backruns on user txs |
| CEX-DEX arb | API access to centralized exchanges + fast private routing | Arbitrage |
| Searcher bundles | Exclusive bundle submission agreements | Liquidations, arb, sandwich |
| Wallet RPCs | Direct wallet routing | All types |
Countermeasures
- BuilderNet: TEE-based shared building where order flow can be submitted to a collective rather than a single builder, redistributing marginal value.
- Relay block merging: losing builders can contribute non-contentious transactions and earn revenue even without winning the auction.
- LUCID/encrypted mempools: universal encryption removes the information asymmetry that makes exclusive flow valuable.
- ePBS: eliminates relays and relay bid adjustment; does not directly address builder concentration.
Open Questions
- Will ePBS reduce the exclusive flow flywheel, or will it just shift the concentration to the ePBS builder auction?
- Can BuilderNet’s shared building model break the flywheel if it achieves sufficient scale?
- How does relay merging change the economics of losing: does contributing flow to the winning block erode the incentive for exclusive routing?
Related Pages
- PBS and MEV-Boost — Architecture of the current PBS system
- BuilderNet and Decentralized Block Building — Shared building as a countermeasure
- Relay Block Merging — New revenue path for non-winning builders
- MEV Auction Design: Open vs. Sealed, Timeboost, and Kairos — Auction design and its impact on revenue extraction
- Private Order Flow and OFA Mechanisms — The OFA and wallet-side view
Key Sources
- Economics Align the Incentives (2026) — flywheel analysis; relay bid adjustment; 84% figure
- Three Years of PBS (Dec 2025) — structural gap taxonomy; relay economics
- Ethereum: A Counterparty Without Counterparty Risk (2026) — 400% premium; robustness framing