Preconfirmations: On splitting the block, mev-boost compatibility and relays

Thanks to @FabrizioRomanoGenove, @meridian and Philipp Zahn for helpful comments and feedback on this post.

:question: What is a Preconfirmation?

There have been a lot of variations on the definition of preconfirmation going around recently in the Ethereum community. In this post we will keep the definition as simple and broad as possible in order to generate the least amount of confusion and avoid arguing on semantics as much as possible:

We call a preconfirmation mechanism any mechanism that ensures (non-positional) inclusion of a (bundle of) transaction(s), if execution is successful, in a finite and bounded amount of time from the emission of the preconfirmation.

:mag: XGA-Style Preconfirmations

We will analyze a specific kind of preconfirmation mechanism – as hinted to in this post on ethresearch – that we came up with some time ago and have been building since then:

An XGA-style preconfirmation mechanism is a preconfirmation mechanism that guarantees (non-positional) inclusion of a sized bundle of transactions in the bottom portion of a predetermined block to be minted 2 epochs after the preconfirmation was emitted. Maximum bundle size is determined at the time of emission of the preconfirmation.

:scissors: Splitting the Block

Looking at the previous definition, I assume the first couple of questions that would come to mind is “what do you mean exactly by the bottom portion of a block?” and “how is the block to include the bundle predetermined?”. Our idea is pretty simple: Partition the block in such a way to keep a top-of-the-block (ToB)[1], high-priority section, in which traditional builders do their usual thing and is allocated through a traditional mev-boost auction or whatever the relay running it prefers; and a reserved bottom-of-the-block (BoB) section, which will serve as allocation space for preconfirmations. In this design, preconfirmation bundles will be allocated via a separate auction in the form of forward contracts.

:busts_in_silhouette: A Two-Auction Format

As briefly mentioned above, in the XGA-style split-block design, preconfirmations are allocated in a completely separate way from the traditional mev-boost auction, allowing them to coexist without excessively disrupting the ecosystem. Traditional builders will be able to do their own thing with minimal adjustments, while everyone else can still enjoy the benefits of preconfirmations.

In simple terms: An XGA-style BoB auction is a multi-unit auction selling gas tokens for a specific block B in fixed-size units (e.g. 100 K gas). These tokens can then be used to submit a bundle[2] that is guaranteed inclusion in B if execution is successful.

As an example, picture this scenario:

  • :clock2: At the start of epoch N-2 we know that the validator V, serving XGA-style preconfirmations, will be the proposer for the K-th slot of epoch N.
  • :oil_drum: $5$M gas out of the standard 30 M will be auctioned off into 50 gas tokens, each representing a capacity of 100 K gas.
  • :shopping_cart: At some fixed time t before the start of slot K, a multi-unit auction allocating the tokens is run. Aki manages to win 5 tokens for K, for a combined capacity of 500 K.
  • :alarm_clock: Within the deadline fixed at some time d before the end of K, Aki uses the 5 tokens to submit a bundle of size just over 400 K gas.
  • :outbox_tray: In the meantime, other BoB auction winners submit their own bundles.
  • :dollar: At the start of K, a traditional mev-boost auction for 25 M gas is run as usual by all relays, and is won by Bogdan via relay R.
  • :brick: After deadline d is reached and the mev-boost auction is over, the BoB part is assembled and attached at the bottom of the max-25 M block submitted by Bogdan via relay R.
  • :tada: Since Aki’s bundle contained no reverting transactions, it is included without any problem – together with the non-reverting bundles submitted by the other BoB winners – somewhere after the portion built by Bogdan.
  • :satellite: The block for K gets broadcasted as usual.
  • :x: Excess tokens for K that didn’t get spent can no longer be used.

:brick: Who Builds the Blocks, then?

Block building, in the case of XGA-style preconfirmations, is handled by multiple parties:

  • :package: The ToB part is built by traditional mev-boost builders as usual.
  • :gift: The BoB part is assembled by the party running the BoB auction.
  • :brick: Merging the two parts and sending the block over is handled by the relay.

In this setup, the relay takes on more work and responsibilities than it currently does. We will explore a potentially beneficial approach to this change later.

:money_with_wings: What Are the Economic Advantages of Preconfirmations?

Well… In general, for the whole range of designs that are being discussed right now this is not clear yet! Conjecturally, some of the proposed preconfirmation mechanisms will allow more value to trickle down to validators, but since the preconfirmation design landscape is so broad and confused right now it’s hard to take into account all the possible market effects that could come out of such designs. For example, most of the preconf mechanisms currently being discussed are pretty unfriendly towards what has been one of the main APY-cows for validators since the dawn of mev-boost: competitive builder/searchers.

:game_die: Why Are We Betting on XGA-Style Preconfs?

It seems clear to us that reserving a spot for non-priority-sensitive transactions can offer several benefits:

  • Users and platforms (e.g. rollups) that are not involved in competitive building/searching just doesn’t care about running HFT operations on L1 can greatly benefit from separating their concerns from those of competitive builder/searchers.
  • On the other end, it eases some of the pressure on the competitive builder/searcher side by removing some of the burden of having to include “filler transactions” to keep their blocks competitive. E.g. freeing them from needing to include blob-bearing transactions that could negatively impact latency.
  • It makes actually pricing inclusion preconfirmations simpler, since it is still regulated by the usual gas pricing model, and at the same time the preconf inclusion market is kept separate from the traditional priority market for position-sensitive transactions.
  • Moreover, we believe in gradual change, allowing time for everyone to adapt to and observe the effects of new, potentially disruptive features in a controlled manner. A split-block design compatible with traditional mev-boost block building offers a less intrusive path to adoption.

:bulb: Rethinking Relays

At the moment running a relay naively is mostly a non remunerative gig. Under XGA-style preconfirmations, the relay does significantly more work and takes on more risk than before, e.g. if a block is missed and/or already sold preconfirmation tokens end up not getting included due to the relay malfunctioning, whoever bought them incurs an active loss of assets. While this sounds scary, it is also a good opportunity to rethink the role of relays in the Ethereum ecosystem.

:shield: Insurance and Reward Mechanisms for Relays

What we are proposing is that a relay can subscribe to an XGA-style preconf platform by staking a collateral that could be used to offer the damaged parties a refund in case of the relay malfunctioning, while sharing a percentage of the platform revenue each time it submits a successful block that includes XGA-enabled preconfirmations[3].

:mega: Introducing XGA


XGA – eXtensible Gas Auctions – is the first L2 platform for XGA-style preconfirmations (lol), designed and built by the combined efforts of Manifold Finance and 20Squares. We’re very willing to make this an open and collaborative effort, so if you have any feedback and/or are interested in building this together with us, please reach out!

Right now we have released on mainnet our v1.0 (yes, this is not a beta, we’re ready to go and currently onboarding validators), with the caveat that in v1.0, the ToB mev-boost auction can only be run on a single relay. We’re currently working on shipping v2.0, which will allow a relay-agnostic auction to be run in the ToB part. You can find more about it at docs.xga.com.


  1. We have specific terms for ToB and BoB auctions, namely α and β-auctions respectively. ↩︎

  2. Note that this doesn’t exclude the possibility of overwriting an already submitted bundle, if re-submitted before the deadline. ↩︎

  3. We are already iterating on designs for captive insurance mechanisms for XGA-style platforms. We will upload a new post detailing some of the possible designs soon. ↩︎

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What happens to the gas tokens for slot N if the proposer of slot N ether fails to propose a block, propose an invalid block, or the block at slot N later gets reorg by honest validators?

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Nice question! If it’s a relay problem then the insurance mechanism should cover that. If it’s a validator problem there’s several avenues, e.g. restaking and/or other kinds of insurance mechanisms. As for reorgs, the design allows locking funds for some time and refunding the victims in case of a reorg. (I’m sure there are other ways but at least we have one possible solution)

As I understand the researcher pays some reward to the relay to add his pre confirmed transaction, which is a great solution for solving a big problem of MEV which is the relay rewards. Why does not the relay just return the reward to the researcher in case the bundle does not get included in the block just like builders do currently? I am not sure if there is need to stake something.

It is also possible for the researcher to send multiple pre confirmed bundles to the many relays, that is the current behavior of builders sending their blocks to multiple relays.

Hi, thanks for the questions!

Yes, indirectly as part of a revenue share agreement between the validator and the relay, and enforced by the preconfirmations platform.

Several reasons, I’ll briefly list a couple:

  • An inclusion preconfirmation in the form of block space forward contracts (XGA-style), and preconfirmations in general I’d argue, are selling not only inclusion rights but timely inclusion rights. Therefore by not including at slot K a preconfirmed bundle that was settled to be included at slot K, the relay or whatever party is responsible for this is actively not upholding its end of the deal and actively hurting the client. This is the whole point of preconfirmations, precommitments or whatever people call this family of products nowadays.
  • The gas tokens can be resold on a secondary market, traded, etc. Imagine having bought gas tokens for slot K on the secondary market after a spike in congestion after it was sold on the auction for way cheaper, now imagine the relay failing on delivering the block containing your bundle and getting refunded at primary market value, this would be pretty bad, right?
  • In this design, the relay is not actually selling the preconfirmations product but rather a secondary platform which then uses several different relays operated by third parties as delivery mechanisms. The relays do some work, take on some risk to be credible and get some money out of it, it’s as simple as that.

In this design the preconf bundles are not sent directly to the relays, rather they’re sent in advance to the preconf platform that sold the bundles, which then distributes it to ALL relays eligible for that (validators willing to actively engage with such a preconfirmations platform on the sell side have to prioritise eligible relays only, obviously) before the slot starts. Then the relays take care of merging the bundles at the bottom of the block sent by the builders. This design is possibly beneficial for both builders and validators for the reasons I have listed in the OP.

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