A simple, small, mev-boost compatible preconfirmation idea

Disclaimer: This post will not contain any nice images, because I am artistically inept.

The reasons why I’m writing this are the following:

  1. Preconfs are a very hot topic right now and many people are working on them;
  2. As usual, some of the proposed solutions advocate for punching changes all the way into the main Ethereum protocol. I’m personally not a fan of this, since life is already full of oh my God, what have I done?™ moments and more drama™ is the least thing everyone probably needs.
  3. MEV-boost is probably the only thing this community has really almost universally agreed upon since MEV has been a thing. So I’d very much try to preserve backwards-compatibility with MEV-boost and generalize on this than coming up with more innovative ways to balkanize our ecosystem even further.

A primer on MEV-boost

This section exists just so that everyone is on the same page. Feel free to skip it or to insult me if you think I summarised things stupidly.

In layman terms, MEV-boost works like this:

  1. Proposer polls the relayer(s) for their best blocks;
  2. Relayer(s) send their best block headers to proposer;
  3. Proposer picks the best block by comparing the block headers received and the block built in-house.
  4. For an in-house block, proposer just signs and broadcasts. For a mev-boost block, proposer signs the header. Relay will broadcast the complete block revealing the payload.

This mechanism is nice because the only party that builders have to trust is relayer: Proposer cannot unbundle blocks and scam builders.

The actual idea

The idea I have in mind works towards extending mev-boost by allowing for preconfs (and most likely for a lot of other stuff if one wants to). Notably, it does not change points 2,3,4 in the previous section, but only point 1.

Suppose proposer has a stash of preconfed txs on the side. The only thing the idea assumes is the following:

By the time Proposer starts polling, it needs to have a finalized lists of preconfed txs to include.

The reason for this will become clear shortly. Having this list at hand, proposer sends a signed JSON object to the relayer when it polls, containing the preconfed txs. This object could look, for instance, like this:

    proposer: address,
    slotNumber: int,
    gasUsed: int,
    blobsUsed: int.
    mergingPolicy: int,
    mustBeginWith: txBundle,
    mustContain: txBundle,
    mustOmit: txBundle,
    mustEndWith: txBundle,
    otherStuff: JSON,
    signature : signature

This design is just an idea. It is by no means fixed yet and most likely can be improved upon both in conceptual and performance terms, so take it with a grain of salt.
The fields proposer and slotNumber are obvious. The fields mergingPolicy, mustBeginWith, mustContain, mustOmit, mustEndWith can all be empty: They contain bundles of transactions that must (or must not) be included in the block. These fields are, effectively, the ones that proposer can use to signal relayer that 'hey, I need the block to respect these requirements, because of previous agreement I made with other parties."

How the proposer comes to define this json object is not our concern, and is outside of the scope of this idea. Just for the sake of clarity though, let’s consider some examples: For instance, XGA, one of the projects 20[ ] is contributing to, provides preconfs as tokenized bottom-of-block space. As such, XGA-style preconfs will produce objects where only mustEndWith is not empty.

The fields gasUsed and blobsUsed tell the relay how much gas and blobs the ‘preconf space’ already claimed. otherStuff exists to be able to extend this standard in the future without more drama™.

Merging policies

The mergingPolicy fields instructs the relay about how to deal with all this information. This is fundamental because, in the end, the relay will still run a traditional mev-boost auction for the remaining blockspace. As soon as a block is built by more than one party there’s a risk that different parties may step up on each other’s toes. As such, mergingPolicy serves as a well-defined conflict resolution policy. If you need a mental reference, think about git conflicts and automated ways to solve them if you so like.

How to define merging policies is up for debate. The community could agree on a common repository where merging policies are defined, voted and agreed upon, and where merging algos are explicitly provided. So, for instance, one merging policy could be:

If the payload coming from the builder contains a transaction that also appears in the preconf bundle, deal with it in the following way:

As said above, XGA sells BOB as preconfs, and leaves TOB open for traditional mev-boost auctions. As such, it has already defined and implemented a merging policy for its bottom of the block case, which will hopefully be open sourced soon.

What does the relay do?

This is probably already kinda clear at this point, but to make it explicit: The relay receives this signed JSON object when the proposer polls. What should it do with it? First of all, it should make some of these fields public to the builders, such as mergingPolicy, gasUsed, blobsUsed and mustOmit. This way builders will know what they can build.

When a block from a builder is received, the relayer will unbundle the block and apply the merging policy to merge it with the preconfed txs. The relay will sign the block header, and send it to the proposer.

From the POV of a builder, everything is kinda the same. They create their block using the info provided by the relay (in the simplest case this just means using slightly less gas than limit), and submit it as their bid.

From this point on, everything works as in traditional MEV-boost.


Ok, so let’s run a rapid analysis of this thing.


  1. Changes to MEV-boost proper are really minimal. We just need to define an API that MEV-boost must listen to to build the polling payload, and redefine the polling logic.

  2. Very little work from Proposer’s side. More work may be needed depending on the preconf system a given proposer wants to use, but then again this is out of the scope of this idea.

  3. Very little work from builder’s side unless people go overly crazy with merging policies. I do not think this is necessarily a problem tho as an overly deranged merging policy would result in builders not submitting anything, and most likely in relayers not taking bets in the first place. So I’d bet that this could pretty much evolve as a ‘let the markets decide’ thing.

  4. This idea is straightforwardly backwads-compatible with traditional MEV-boost: If the polling payload is empty, we collapse to a traditional MEV-boost auction with no other requisites.

  5. This idea allows for gradual phasing out of MEV-boost if the community so decides. For instance, proposers may agree to produce bundles where usedGas is a very low parameter in the beginning (it won’t exceed 5M for XGA, for instance), meaning that the majority of blockspace would come from traditional building, with only a tiny part being preconfs or more generally ‘other stuff’. This parameter may then be increasingly crancked up or varied with time if the community so decides, effectively phasing out traditional block building in favor of ‘something else’. In this respect yes, I know I’m being vague here but when it comes to how this thing could be adopted I can only speculate.

  6. This system can be extended in many ways, and it is flexible. Merging policies could be defined democratically, and the polling info could be extended effectively implementing something akin to PEPSI, for instance. Another possible extension/evolution can be using otherStuff to define Jito-style auctions. I mean, there’s really a plethora of ways to go from here.

  7. The polling payload is signed by the proposer, and the block header is signed by the relayer. This keeps both parties in check as we accumulate evidence for slashing both. For instance:

    • Imagine I get some preconf guarantee from proposer and that I have evidence of this. Again how this happens is outside of the scope of this post, as this mechanism is agnostic wrt how preconfs are negotiated.
    • Now suppose furthermore than my preconfed tx does not land in the block.
    • I can use the chain of signed objects to challenge both relayer and proposer. If my tx wasn’t in the polling info signed by proposer, that’s proposer’s fault. On the other hand, if it was, but it wasn’t in the block, then it’s relayer’s fault. I think this is enough to build a slashing mechanism of sorts, which could for instance leverage some already available restaking solution.

    Note: If there’s enough interest in this idea, we as 20[ ] can throw some open games at it and simulate the various scenarios. Let me know!

  8. Ethereum protocol doesn’t see any of this. So if it fucks up, we just call it a day and retire in good order without having caused the apocalypse: Relays will only accept empty payloads, proposers will only send empty payloads, and we’ll essentially revert to mev-boost without anyone having to downgrade their infra. I think this is the main selling point of this idea: The amount of ways to make stuff explode in mev-related infraland are countless, so this whole idea was built with a ‘it has to be failsafe’ idea in mind.


  1. Relayer must unbundle builder blocks to do the merging. I do not think this creates a huge trust issue as relayer can already do this as of now: In general, a relayer that scams builders is a relayer that won’t be used again, and will go out of business quickly.

  2. Relayer must do computational work. This is probably the major pain point. This idea entails slightly more latency, as an incoming bid cannot be relayed instantly because mergingPolicy has to be applied. The computational penalty is furthermore heavily dependent on how deranged the merging policy is. As a silver lining, this computational work is provable as both the merging info and the resulting block are signed. The result is that we have provable evidence to remunerate a relay for its work if we want to, possibly solving a major pain point for relayers in traditional mev-boost.

  3. Relayer is slashable if it screws up. Again, how this should be implemented is outside of the scope of this idea as this mechanism only accounts for the needed trail of evidence to implement slashing, but does not deal with the slashing per sé. Anyway, it is still worth reasoning on the possible consequences of this: If slashing policies are implemented, Relayers will most likely need to provide some collateral or implement some form of captive insurance. Again, this may signify more complexity on one hand but also opportunity on the other, as relayers may for instance decide to tokenize said collateral and develop mechanisms to make money out of these newly created financial instruments. As relayers are private enterprises I’ll leave these considerations to the interested parties.

  4. Polling info must stay fixed. This is related to point 3 above and point 6 of the Pros subsection: If the polling info changes all the time, this means huge computational stress for the relayer, and it furthermore allows for malicious behavior from the proposer: For instance, a proposer could send two different polling payloads, and include a given preconfed tx only in one of them. How to resolve these inconsistencies is an open question. In my opinion, the wisest and simplest thing to do would be requiring the polling info to be fixed, meaning that if proposer signs conflicting payloads for the same slot this should be considered akin to equivocation, and thus a slashable offence.

    By the way, the consequence of this is that the idea proposed here necessarily excludes some preconf use cases. This is related to my comment here and I think it is unavoidable if we want to keep MEV-boost around. As the majority of revenue from MEV comes precisely from the bids of very refined, high-time frame searchers, and as I am quite sure that validators don’t want to give this money up at least for now, ‘leaving these players be’ by ruling out such preconf use-cases is in my opinion the most practical option, and exactly the rationale motivating this idea.

Closing remarks

That’s it. If the idea is interesting enough let me know, I’ll be happy to start a discussion around it. The 20[ ] team will also be around at EthCC if you want to discuss this in person.


How much latency do we think this add to the overall block release pipeline? The additional steps are:

  • The relayer sends the requirement list to the builder.
  • The builder sends the block to the relayer.
  • The relayer verifies the block (not necessary if it’s an optimistic relayer).
  • The relayer responds with the header back to the proposer.

Alternatively, we could assume the relayer can unbundle the block and fill in the requirement list itself without having to go through the builder.


Yeah sorry, I wasn’t being clear. We need to absolutely assume that “the relayer can unbundle the block and fill in the requirement list itself without having to go through the builder.” if we don’t want builders to MEV the sh*t out of the preconfs, lol.

So to answer you more extensively:

The relayer sends the requirement list to the builder.

This is only needed depending on the merging policy. For instance, for a ‘BOB, low priority’ kind of preconf there is very little we need to do. Basically the only thing builder has to know is what the merging policy is. For instance, to make everything simpler for builders XGA uses things such as:

If builder submits a tx which also appears as a preconf tx in BOB, strip it out of BOB.

This means that for builders nothing really changes, they have priority over preconfed txs. In theory a competitive builder may fear that a competitor could use preconf to backrun their block, but given that preconfs have to be ready before the auction starts, I think this is a minimal concern. In general, we’re talking about two very different markets here.

As I said in the main post, the more deranged the merging policy is, the more complicated and terrible the mechanism becomes. Luckily for us the market is often very swift in punishing this kind of inefficiencies, and a very deranged policy would probably fall out of fashion rather quickly!

The builder sends the block to the relayer.

Nothing new under the sun! Again, builders may need to do extra work if the merging policy requires it, which is why they have to be picked wisely.

The relayer verifies the block (not necessary if it’s an optimistic relayer).

This is the pain point I was talking about before. Verifying the block is basically necessary, as the relayer is the party doing the merging. So it cannot blindly trust the builder. For instance, as hinted above, if the same tx (the literal same tx, same nonce, same everything) is submitted both by builder and as a preconf, the relayer will have to actively avoid that the tx shows up as duplicated in the block. This extra computational work is the main source of added latency. The more the merging policy is complicated, the worse it gets.

The relayer responds with the header back to the proposer.

Nothing new under the sun here. The main difference is that the relayer signs it, accruing the chain of evidence that can be used to implement slashing.

1 Like

If I understand correctly, the idea is to allocate a pre-confirmed portion of block space for proposers to include transactions.

Proposers can receive open market payments through MEV-boost relays, ensuring fairness by prioritizing these transactions top-of-block (TOB).

This approach increases metered block space for both the open market and preferred routes (pre-confirmation agents), balancing the negative externalities of frontrunning using the pre-confirmation feature.

From a proposer’s standpoint, they benefit from participating in a dual market scheme while maintaining the flexibility to optimize their proposer payments.

This enables optionality to the existing block space market without requiring any breaking changes.

1 Like

If I understand correctly, the idea is to allocate a pre-confirmed portion of block space for proposers to include transactions.


Proposers can receive open market payments through MEV-boost relays, ensuring fairness by prioritizing these transactions top-of-block (TOB).

This depends. If you use the mustBeginWith field in the payload, and use a mergingPolicy which privileges preconfs over traditional MEV-boost, this may not be true anymore, as preconfs would get TOB. It really depends on how one runs the system but yes, I agree with you that what you wrote is probably the most obvious and wisest way to run it, lol.

From a proposer’s standpoint, they benefit from participating in a dual market scheme while maintaining the flexibility to optimize their proposer payments.
This enables optionality to the existing block space market without requiring any breaking changes.

Yes. This is the main point of the idea. Also, this results in very little changes to be implemented from the proposer’s POV. Ironically, Vitalik is giving a talk today called ‘Don’t overload the proposer’, I guess at least my timing was right :stuck_out_tongue:

1 Like

as far as my understanding goes, builder will build a block as usual, relayer will unbundle the block and put the preconfed tx in it and send it to proposer, and proposer would propose the mev-boosted block.

thus almost no changes to other roles but for relayer?

forgive me for my lack of knowledge, but i have 2 rly basic questions
i) if preconfed txs conflict with already-in txs in the builder’s block, wouldn’t that harm the builder’s profitability?
ii) if the builder’s block arrives full, then wouldn’t there be times when relayer cannot add more txs to the block?


See Fabrizio’s answer above:

There is a reserved space in the block for this. If a builder block arrives full there’s no problem since they’re occupying space that doesn’t intersect with the space occupied by the preconf bundles.


Adding to @dpl0a answer:

i) It really depends on the merging policy. Merging policy, as I stated in the original post, can be pretty much everything, including a mechanism that completely destroys the builder’s profit. For instance, it could be:

Disregard builder transactions altogether.

Clearly, this is not wise nor profitable. In general, clever merging policies should ensure builders’ profitability, lest proposer sees its revenue declining.

ii) This is why I suggested to have gasUsed and blobsUsed in the polling objects. These items should be propagated by relayer to builders to instruct them about what to build. Again, I’d expect that the wise thing here is converging on some agreed-upon values. For instance, XGA at the moment caps gasUsed at 5M. In this case a builder submitting a block using more than 25M is automatically disqualified from the auction.

One important thing I probably did not stress enough in the main post is this: I’m assuming preconfs and MEV-boost building to be, at least for now, completely different markets. In my intuition, the main preconfs users are probably rollups, with sequencers that want to give inclusion guarantees ASAP. This is a completely separated usecase from competitive building (arbitrages etc.) so I wouldn’t expect much conflict from these two markets at the merging phase. I’m ready to be proved wrong though!

1 Like