MEV-Boost and PBS: How Builder Auctions Affect Your Trades

Every time you submit a trade on a DEX, your transaction does not go straight to a miner or validator anymore. It goes through an auction. Specialized builders compete to pack your transaction into a block alongside their own MEV strategies, and the winner pays the validator for the right to decide what your trade looks like once it lands on-chain. This system, called MEV-Boost, runs almost all of Ethereum's block production today, and understanding how it works helps you predict when your trades are more likely to get sandwiched, delayed, or executed at a worse price than expected.

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What MEV-Boost and PBS Actually Do

Proposer-Builder Separation (PBS) splits Ethereum block production into two jobs. Validators (proposers) pick which block to publish. Specialized builders construct the actual block, competing to include the most profitable set of transactions.

MEV-Boost is the software that connects these two sides. Built by the Flashbots Collective and active since the Merge in September 2022, it lets a validator auction off block-building rights to competing builders through intermediary relays. The MEV-Boost block auction contributes approximately 90% of all Ethereum blocks.

The flow works like this:

• Your wallet broadcasts a transaction (either publicly or through a private RPC).
• Searchers and builders scan for profitable opportunities, including yours if it creates arbitrage or sandwich potential.
• Builders assemble full blocks and submit bids to relays.
• Relays verify the bid is valid and forward the highest-paying block header to the validator.
• The validator signs the header blind, without seeing the transaction contents, and the winning builder reveals the full block.

That blind-signing step matters. Validators trust relays to have checked the block honestly. This trust requirement is exactly what critics point to when they call MEV-Boost a centralization risk rather than a clean solution.

Image source: boost.flashbots.net/

Why Builder Concentration Should Worry Active Traders

The theory behind PBS was that competition among builders would keep the market fair and efficient. In practice, a handful of builders now dominate block construction, and that concentration directly shapes your trade execution.

Recent relay data shows the scale of this concentration. Over a 24-hour window at the end of August 2026, Titan built 47.64% of blocks, BuilderNet built 19.25%, and Quasar built 18.01%, with the rest split among smaller builders like Eureka and bombora. build. On the relay side, relay.ultrasound.money handled 35.26% of payloads; titanrelay.xyz handled 27.99%, and bloxroute.regulated.blxrbdn.com handled 25.40%.

This isn't a new trend. Between October 2023 and March 2024, just three builders, beaverbuild, rsync, and Titan, produced approximately 80% of all MEV-Boost blocks. Independent researchers measured the builder market's Herfindahl-Hirschman Index at approximately 3,892, well above the threshold that antitrust regulators typically consider highly concentrated.

Vertical integration is accelerating this. Titan operates both a leading builder and a leading relay, which lets it skip the trust problem entirely for its own order flow. Relay market share has shifted steadily toward Titan Relay in recent months, largely at the expense of bloXroute and Ultrasound.

For a trader, this concentration means two things:

• Fewer entities decide transaction ordering. If your trade interacts poorly with the dominant builder's strategies, there is no meaningful alternative pathway most of the time.
• Censorship risk rises with concentration. A small number of builders complying with sanctions lists or regulatory pressure can meaningfully change which transactions get included quickly and which get delayed.

If you want the deeper mechanics of how this value gets extracted from your specific trades, What Is MEV in Crypto and Is It Stealing From You? breaks down the searcher-level tactics that builders package into these auctions.

How Builder Auctions Affect Your Actual Trade Execution

Builder auctions change three things about your trade that a simple gas-fee model does not capture.

Execution price. Builders order transactions to maximize their own bid. If your swap creates a profitable sandwich opportunity, a searcher will bundle a front-run and back-run around it and pay the builder for placement. You still get filled, but at a worse price than the pool showed when you clicked confirm.

Inclusion timing. Public mempool transactions increasingly get deprioritized. Over 90% of arbitrage transactions now route through private channels rather than the public mempool. If your wallet or aggregator sends transactions publicly instead of through a protected RPC, you are exposed to searchers scanning the mempool in real time, and your inclusion can also be slower during high-demand blocks because builders prioritize bundles that pay more.

Which trades get included at all. Builders are not obligated to include your transaction. In practice, this rarely blocks ordinary trades, but it means censorship is a structural possibility, not a hypothetical one. The share of OFAC-compliant blocks on Ethereum has fluctuated dramatically since the Tornado Cash sanctions in August 2022, moving from a large majority to a minority depending on which relay validators choose. That swing shows the filtering choices of a small number of intermediaries can move network-wide behavior.

Image source: relayscan.io

Protocol Comparison: How the Major Relays Differ

Not all relays behave the same way, and the one your validator or RPC provider uses affects your trade in practical terms.

The practical takeaway: if you're using a wallet or RPC provider with private transaction protection, check which relays and builders it routes through. A provider that only submits to censoring relays can add delay to certain transaction types during periods of regulatory sensitivity.

Things to Check Before You Rely on a Private RPC or MEV Protection Tool

Before you assume a "protected" transaction route actually protects you, verify:

• Whether the RPC sends to multiple builders or just one, since single-builder routing recreates the centralization problem at the wallet level
• Whether the service publishes real-time relay and builder statistics you can audit
• Whether the tool actually prevents sandwich attacks or just hides your transaction from the public mempool without changing builder incentives
• Whether the provider has any history of dropped or unusually delayed transactions during high-MEV periods
• Whether the tool charges a fee structure that scales with trade size, which can erode the protection's value on smaller trades

For a full breakdown of specific tools that address sandwich attacks directly, Best MEV Protection Tools to Stop Sandwich Attacks on DEX Trades compares the leading private RPC and transaction-protection options in more depth.

Where Ethereum Is Heading: Enshrined PBS

The Ethereum Foundation's response to builder centralization is to move PBS into the protocol itself, a change called enshrined PBS, or ePBS. EIP-7732, the enshrined PBS proposal, is one of two headline changes in the Glamsterdam hard fork, targeting activation around the end of August 2026. The goal is to internalize the relay's trust function directly into the consensus layer, removing the need for external intermediaries that validators currently have to trust.

Alongside ePBS, researchers are pushing inclusion list mechanisms (sometimes called FOCIL) that would let validators force certain transactions into a block regardless of what the winning builder wants, directly targeting the censorship risk described above.

None of this changes trade execution overnight. Builder concentration and MEV extraction will not disappear because the trust model moves on-chain; a dominant builder with better infrastructure and order flow can still win most auctions under ePBS. What changes is who you have to trust and how enforceable the rules are.

Common Mistakes Traders Make With MEV-Boost Dynamics

• Assuming a private RPC eliminates MEV risk entirely, when it usually just removes public mempool exposure and still routes through the same concentrated builder market
• Ignoring slippage tolerance settings, which is the single easiest lever a sandwich bot exploits regardless of which builder wins the block
• Trading large size on volatile pairs during periods of high network congestion, when builder competition for bundle placement is most intense
• Assuming all "MEV protection" branding is equivalent, without checking whether the tool actually reroutes away from sandwich-friendly builders
• Overlooking that inclusion delays during regulatory news cycles can be a censorship signal, not just network congestion

My Take

Builder concentration is the single most underappreciated risk in DeFi trade execution right now. Titan's roughly 47 to 50 percent share of blocks, combined with its vertical integration into its own relay, means a large share of Ethereum's transaction ordering runs through one commercially motivated entity. That is not a conspiracy theory. It is measurable market share, and it should factor into how much size you're willing to trade without private transaction protection.

For active DEX traders, using a private RPC that routes to multiple non-censoring builders is close to mandatory, not optional, especially for trades over a few thousand dollars. For smaller or infrequent traders, the cost-benefit is weaker, since the fee or complexity of some MEV protection tools can outweigh the sandwich risk on a $50 swap.

I would avoid any protection service that will not disclose which relays and builders it routes through. Opacity here defeats the purpose. And I would not assume ePBS solves this problem when it activates later in 2026. It changes the trust model, not the economics that make builder concentration profitable in the first place.

When Builder Auction Risk Matters Most

• Large trades on volatile pairs: sandwich profitability scales with trade size and price impact, so this is where protection matters most
• Low-liquidity token launches: thin order books create outsized sandwich opportunities for searchers watching the mempool
• High network congestion periods: builder competition intensifies, and inclusion delays for unprotected transactions grow
• Trades routed through the public mempool: any transaction visible before confirmation is exposed regardless of trade size

When It Matters Less

• Small, routine swaps on deep-liquidity pairs like major stablecoin pairs, where sandwich profit rarely exceeds gas costs
• Trades already routed through a reputable private RPC with multi-builder submission
• Layer 2 activity on rollups with different, often more centralized but currently lower-MEV sequencing models, where the economics differ from Ethereum mainnet

Conclusion

MEV-Boost turned Ethereum's block production into an auction, and that auction is dominated by a small number of builders and relays whose incentives do not automatically align with getting you the best trade execution. The practical decision for any active DeFi trader is whether to route transactions through a private, multi-builder RPC, and how much slippage protection to set on top of that. Check which relays and builders any tool you use actually connects to before trusting the label "MEV protected." ePBS will change the trust architecture later in 2026, but builder concentration and the profit incentive behind sandwich attacks will still require active management on your end.

FAQs

1. Does MEV-Boost mean my trade will definitely be sandwiched?

No, most trades on deep-liquidity pairs are not profitable enough to sandwich. Risk rises with trade size, low liquidity, and public mempool exposure.

2. Can I opt out of MEV-Boost as a trader?

You cannot opt out of the underlying block auction, but you can reduce exposure by using a private RPC that limits mempool visibility and routes to non-sandwiching builders.

3. Is builder concentration a sign that Ethereum is becoming centralized?

It reflects real centralization pressure at the block-building layer specifically, not the validator set as a whole, since any staker can still propose blocks. The Ethereum Foundation is actively addressing it through ePBS and inclusion list proposals.

4. Will enshrined PBS eliminate MEV extraction?

No, ePBS changes who you have to trust for block construction, not the underlying profit incentive that drives builders to extract value. Sandwich attacks and arbitrage will still exist after ePBS activates.

5. How do I check which builder or relay actually processed my last transaction?

Public dashboards like relayscan.io let you look up recent blocks by relay and builder, and some block explorers now tag which relay delivered a given block. This lets you audit whether your RPC provider's claims match on-chain reality.

References

Who Wins Ethereum Block Building Auctions and Why?: https://arxiv.org/html/2407.13931v1

MEV-Boost Relay & Builder Stats: https://www.relayscan.io/

Block Builder Glossary: https://www.spark.money/glossary/block-builder

MEV-Boost in a Nutshell (Flashbots): https://boost.flashbots.net/

Ethereum Glamsterdam Upgrade Explained: https://everstake.one/resources/blog/ethereum-glamsterdam-upgrade-explained

Enshrined PBS: The Road to Trustless Ethereum: https://hackmd.io/@astaxxoo/BJZS6jP2Wl

An Analysis of Arbitrage Markets Across Ethereum, Solana: https://academy.extropy.io/pages/articles/mev-crosschain-analysis-2025.html

Ethereum's OFAC-compliant blocks fall to 27%: https://www.theblock.co/post/230179/ethereums-ofac-compliant-blocks-fall-to-27-marking-a-drop-in-protocol-level-censorship

MEV-Boost Market Update (Toni Wahrstätter): https://x.com/nero_eth/status/2004972045516292339

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