What Is MEV-Boost? Builders, Relays and Proposers Explained

By Merunas Grincalaitis, founder of MEV DAO and a developer who builds MEV, sniping and trading bots on Ethereum. Last updated: October 2026.

TL;DR: MEV-Boost is open-source software from Flashbots that validators run next to their consensus client. Instead of building its own block, the validator asks a set of relays for the highest-paying block made by specialized block builders, signs its header without seeing the contents, and gets the payment the builder promised. Around 90% of recent Ethereum blocks came through MEV-Boost when I checked on October 25, 2026. If you run a bot, this is why your transaction ordering depends on builders and bundles rather than gas price alone. ePBS (EIP-7732), planned for the Glamsterdam upgrade, will move this auction into the protocol, but it has no mainnet date yet.

Almost every post on this blog mentions MEV-Boost in passing: bundles go to builders, builders bid for blocks, validators take the best bid. I realized I never wrote down how the pieces fit together. So here it is, from the point of view of someone who sends bundles every day and has to care about who actually builds the blocks.

What is MEV-Boost?

The Flashbots documentation describes MEV-Boost as open-source middleware run by validators to access a competitive block-building market. It's Flashbots' implementation of proposer-builder separation (PBS): the idea that the job of choosing and ordering transactions (building) should be separate from the job of proposing the block to the network (the validator's job).

In practice it's a small sidecar process. Your consensus client (Lighthouse, Prysm, Teku, Nimbus, Lodestar) talks to MEV-Boost through the builder API, and MEV-Boost talks to one or more relays. If no relay answers with a good bid, the validator falls back to building a normal block from its own mempool. Flashbots estimates validators running MEV-Boost can increase their staking rewards by over 60%, which is the main reason almost everyone runs it.

Three roles matter:

Builders collect transactions and bundles from users, searchers and private order flow, assemble the most valuable block they can, and bid for the right to have it proposed.
Relays sit in the middle. They receive blocks from builders, check them, and offer the validator only the header and the bid amount.
Proposers (validators) pick the highest bid, sign the header blindly, and the relay then publishes the full block.

How does a block get built with MEV-Boost?

Every 12-second slot goes roughly like this:

Searchers and users send order flow to builders. Bots send bundles with eth_sendBundle, wallets send private transactions through RPCs like Flashbots Protect, and builders also read the public mempool.
Builders assemble candidate blocks and keep resubmitting improved versions to relays as new transactions arrive during the slot. Each submission includes a payment to the validator's fee recipient.
Relays validate the blocks (that they're valid and that the promised payment is really there) and keep the best bid per slot.
The proposer's MEV-Boost asks every configured relay for a header and picks the highest bid.
The proposer signs the header without seeing the transactions. This is the "blind" part, and it's what stops a validator from stealing a builder's MEV.
The relay releases the full block and broadcasts it. The validator gets paid, the builder keeps whatever margin it left for itself, and searchers whose bundles were included pay what they bid.

That flow explains something that confuses a lot of people on their first bot: a higher gas price in the public mempool doesn't guarantee a better position. The builder decides ordering, and builders sort by what pays them, which is why bundles and direct builder tips exist. I went into the tipping side in sniper bot gas settings.

Why do relays need to be trusted?

Relays are the weakest point of MEV-Boost because both sides have to trust them. Builders trust that the relay won't leak or steal the contents of their block. Validators trust that the relay will actually publish the block after they sign the header, and that the payment is real. The mev-boost README says it plainly: validators should only connect to relays they trust.

This isn't theoretical. On April 3, 2023, a malicious validator exploited a bug in the mev-boost-relay software running on the ultra sound relay and stole about $20 million from sandwich bots. According to the Flashbots post-mortem, the relay would reveal a block's body when given a signed but invalid header, which let the attacker see the bots' transactions and unbundle them. The fix, shipped in mev-boost-relay v1.0.0-alpha3, makes the relay successfully publish the block before revealing its contents.

As a searcher, that episode taught me two things. Sandwich-style bundles that depend on being atomic can be unbundled if any link in the chain misbehaves. And it's worth caring which relays and builders your flow ends up with.

Which relays and builders produce Ethereum blocks today?

These numbers come from relayscan.io, which tracks payloads delivered by each relay, using the 7-day view I checked on October 25, 2026. A block can be offered through several relays at once, so treat these as market shares rather than exact block counts.

Relay	Share of payloads (7 days)	Top builders	Share of blocks (7 days)
Ultra Sound	33.3%	Titan	52.5%
Titan Relay	32.2%	Quasar	21.2%
Aestus	16.3%	BuilderNet	13.4%
bloXroute (regulated)	15.5%	Eureka	7.6%
Flashbots	1.7%	bombora	2.2%
Agnostic	0.8%	Builder+	1.9%

Two things stand out. Block building is very concentrated: two builders produced roughly three quarters of MEV-Boost blocks that week. And the Flashbots relay, which started all of this, now delivers a small slice; Flashbots' own building has largely moved into BuilderNet, the block-building network it operates with partners. On the validator side, Rated's relay explorer showed about 90.2% of slots in the last 500 epochs going through MEV-Boost and 9.6% built locally ("vanilla") when I checked the same day.

What does MEV-Boost mean for bot developers?

If you build searchers, snipers or rescue bundles, MEV-Boost is the environment you're working in:

You send to builders, not relays. Relays only deal with builders and validators. Your bundle goes to a builder endpoint, such as relay.flashbots.net (which despite the name is a bundle endpoint) or the builders' own RPCs.
Send to several builders. Because no single builder wins most slots, a bundle sent to only one builder misses most blocks. Flashbots supports multiplexing through a builders field in eth_sendBundle, and many searchers just post the same bundle to each major builder directly.
Your bid competes with the whole block. Builders include your bundle if it makes their block more valuable than leaving it out. That's why profit splits look so harsh, a point I explained in how MEV bots make money.
Some flow earns refunds. BuilderNet can refund part of what you paid if your bundle added value, which is covered in my Flashbots bundles guide.

The same logic powers wallet rescues. When a sweeper bot watches a compromised wallet, the rescue only works because a bundle that funds gas and moves tokens in one block can reach builders privately, without ever touching the public mempool.

Should a solo validator run MEV-Boost?

Most do, and for good reason: it's where the extra rewards are. If you run it, the README gives you a few settings worth knowing:

-relay / -relays: the relays you connect to. Pick several reputable ones so you aren't depending on one operator.
-relay-check: checks that relays are reachable when MEV-Boost starts.
-min-bid: a minimum bid in ETH. If no relay offers more, your client builds the block locally. Some solo stakers set a small minimum so they build locally when MEV is low, which helps keep blocks built by many different parties.

Relays also differ in policy. Some filter transactions involving sanctioned addresses (bloXroute's "regulated" relay says so in its name) and some don't, so relay choice affects what your blocks can contain.

What is ePBS and will it replace MEV-Boost?

Enshrined proposer-builder separation, specified in EIP-7732, puts the builder auction into the Ethereum protocol itself. Per ethereum.org's Glamsterdam page, it's the headline change of the Glamsterdam upgrade and removes the protocol's reliance on trusted relays: builders commit to blocks on-chain, and the protocol, not a relay, enforces that the proposer gets paid and the payload is revealed.

The timing is still open. Glamsterdam activated on the Sepolia testnet on October 6, 2026, but the Ethereum Foundation announced on October 22 that the Hoodi testnet activation planned for October 26 was postponed to investigate an issue with EIP-8037, and no new date has been set. There's no mainnet date either. Until it ships, MEV-Boost and relays remain how most blocks are made, and even afterwards builders and searchers will still exist. What changes is who you have to trust in the middle.

Frequently asked questions

What is MEV-Boost in simple terms?

It's software validators run that lets them sell the job of building their block to specialized builders. Relays pass the best builder bid to the validator, who signs it blindly and receives the payment.

Is MEV-Boost made by Flashbots?

Yes. Flashbots wrote and maintains the open-source mev-boost client and the mev-boost-relay software. Relays run by other teams, such as Ultra Sound, Aestus and Titan, use that relay code or their own versions.

What is a MEV-Boost relay?

A relay is a trusted middleman between builders and validators. It receives full blocks from builders, checks that they are valid and that they pay what they promise, and gives validators only the header until the block is signed.

What percentage of Ethereum blocks use MEV-Boost?

About 90% of recent slots when I checked Rated's relay explorer on October 25, 2026, with the remaining roughly 10% built locally by validators.

Do I need MEV-Boost to send Flashbots bundles?

No. Searchers send bundles to builders, and MEV-Boost is only run by validators. You need MEV-Boost if you run a validator and want builder blocks.

Will ePBS end MEV-Boost?

It's designed to remove the need for trusted relays by moving the builder auction into the protocol through EIP-7732 in the Glamsterdam upgrade. It has no mainnet date yet, and builders and searchers will keep operating after it ships.

At MEV DAO I build bots that live inside this builder market every day. If you need a searcher or bundle pipeline that sends to the right builders, take a look at custom MEV bot development or message me on Telegram.