TL;DR: Placement commitments extend mev-commit preconfirmations beyond simple inclusion/execution guarantees by allowing bidders to specify where within a block their transaction should be placed. This provides practical protection against reordering slippage and enables precise execution strategies such as top-of-block liquidations or better retail trade pricing. By turning transaction placement into a priced market dimension, placement commitments open a new design space for managing execution risk in Ethereum.

Preconfirmations (i.e., provider commitments) in mev-commit provide transaction inclusion/execution guarantees before block confirmation. Traditionally, these guarantees (once given) are binary: either the transaction is included/executed in a block or the user receives compensation.

We now introduce an extension that allows bidders to express not only whether their transaction should be included/executed, but also where within the block it should be placed. We refer to this as a position constraint in the bid and the corresponding commitment as a placement commitment, achieving better trade pricing and execution quality both for retail and sophisticated traders.

Motivating Use Cases

Transaction ordering in Ethereum has tangible economic consequences. Recent empirical work [1] on decentralized exchanges showed that reordering slippage costed Uniswap users $9m in losses over a period of two months, with losses concentrated among the transactions that are positioned within 10–25% after the top of a block. Placement commitments directly mitigate this effect: by bidding for inclusion in the top 10% of a block, users can bound their exposure to adverse execution, aligning transaction costs more closely with the actual risk of reordering. These benefits can be realized by end users as dapps and wallets use the mev-commit network to privately send their transactions and bid for high placement.

For specialized actors such as searchers and solvers, precise placement grants greater execution and risk management. Arbitrage, liquidation, and backrunning strategies often require narrow execution conditions—for instance, execution at the end of the block or absolute top-of-block inclusion for a liquidation. Placement commitments allow these actors to request such guarantees explicitly, reducing failed attempts, contract costs and gas, and enabling a market for pricing positional certainty.

Together, these use cases highlight why positional guarantees are not merely technical conveniences but essential tools for more efficient execution in Ethereum’s block-building pipeline.

Mechanism Description

We extend the bid format in mev-commit with a new field, bidOptions, that allows bidders to specify ordering requirements, see mev-commit docs. These can be defined in different ways:

• Absolute position

  • e.g., “transaction must be the first in the block.” or

  • “transaction must be among the first 7 transactions in the block.”

• Relative to block length / transaction index percentile

  • e.g., “transaction must be in the top 10% of transactions the block” or

  • “transaction must appear in the bottom 5%.”

• Relative to gas usage / gas usage percentile

  • e.g., “transaction must be within the first 10% of total gas used in the block.”

Relative constraints are useful since the total number of transactions in a block is typically not known beforehand, and it allows, e.g., to avoid the high slippage costs incurred by being placed after the top 10% of the block as discussed above and analyzed in [1]. Placement relative to gas usage is more robust against manipulations, e.g., by filling up the block with lots of cheap transactions that place most relevant transactions in the top 10% of the block. By asking for inclusion within the first 10% of total gas, a bidder ensures that their transaction executes before gas-heavy operations that may significantly alter the block’s execution state.

Bidders can also combine expressions to convey their requirements more precisely. E.g., they can request the transaction to be within the top 5% of total gas used and additionally among the first 10 transactions in the block. In case of transaction bundles, a set of constraints is considered satisfied if each constraint is satisfied by at least one transaction in the bundle. For example, a bidder can submit a bundle of 10 transactions and request the bundle to be placed among the first 7 transactions in the block, which counts as satisfied as long as the first transaction in the bundle is within the top 7 transactions.

Offering different ways to express the constraints allows bidders to find the optimal way to state their requirements without putting unnecessarily strict constraints on the block builders. Overall, these new position constraints shift preconfirmations from inclusion/execution only guarantees to placement-aware guarantees and enable additional use-cases.

Placement Commitments as State-Locks

Placement commitments can be used to lock state, fully or partially. Note that the state of Ethereum depends on all prior transactions from previous blocks and within the current block. Thus, the first transaction in a block directly builds upon the state determined at the end of the previous block. This means that getting the guarantee that a transaction gets included as the first transaction of the next block implies that the current state of the chain gets locked and this transaction gets executed based on this state. This gives users such as searchers the same guarantees as state-lock auctions proposed on Ethereum Research [2].

Such state locks give certainty to searchers and are useful in many scenarios: when there is an arbitrage opportunity, locking the state ensures that the full opportunity can be captured by the searcher without interference from others. Similarly, when a searcher detects a liquidation opportunity, locking the state ensures that nobody else can trigger the liquidation beforehand. Applications are discussed in more detail below.

While top-of-block commitments provide great guarantees to users, block builders likely charge substantial premiums for such commitments since there can only be a single transaction at the absolute top of the block. A more light-weight and likely cheaper alternative is requesting placement near the top of the block, without requesting the absolute top. This does not completely lock the state, but limits the possible state drift as statistically shown[1]. E.g., if a swap is guaranteed to be among top three transactions in a block, there can be at most two other transactions prior to it, and unless one of the two touches the exact same token pair on the same liquidity pool, the swap is unaffected by the extra transactions. The further down transactions are allowed to be placed, the weaker the guarantees become at an increasing discount. Overall, the mechanism gives flexibility to searchers and trading venues alike to trade between price and execution guarantees.

Examples of Potential Applications for Placement Commitments

Among others, the following use cases are examples that benefit from placement commitments:

• DEX trade protection: Traders on decentralized exchanges are highly sensitive to reordering slippage, where the execution price depends on their transaction’s relative position. By requesting top-of-block or top-10% placement, a user or the dapp on its users’ behalf can bound the amount of state drift that can occur before their swap executes. This reduces the risk of receiving a worse price due to intervening trades and ensures costs are more closely aligned with actual reordering risk.

• (Blind) backrunning: A searcher expects heavy net buying of token T in the current block (e.g., due to a catalyst). To harvest the price impact of those buys, the searcher requests bottom-of-block (or bottom-10%) placement for a sell transaction. This positioning makes it likely (though not guaranteed) that most buy transactions in the block land before the searcher’s trade, letting them backrun the aggregate order flow and sell into the locally elevated price. Placement commitments thus convert “hope I end up last” into a priced guarantee about where in the block the backrun lands, improving expected profitability.

Discussion and Conclusion

Placement commitments extend the design space of preconfirmations by making block position a priced dimension. They can be understood as a state-bounding mechanism: i.e., they reduce exposure to state drift and reordering effects by constraining where in the block a transaction is included/executed.

Several research questions arise from this comparison:

• How should markets price placement guarantees?

• To what extent can placement commitments serve as a substitute for explicit state-locks for a particular state in practice?

• What are the trade-offs between efficiency, complexity, and determinism across these approaches?

By framing transaction placement as a marketable property, placement commitments offer new tools for users, searchers, and protocols to better manage reordering slippage and execution risk. At the same time, they highlight the need for deeper analysis of the economics and system-level implications of trading ordering guarantees in blockspace.

Go hands on today! Check out the mev-commit documentation to start placing trades precisely. Primev welcomes all forms of feedback and requests for new features including further extending the expressiveness of bids.

References

[1] Evan Kim, Murat Akdeniz. Trading in the Dark: Measuring Reordering Slippage in MEV. Primev mirror. 2024

[2] dmarz. State Lock Auctions: Towards Collaborative Block Building. Ethresearch 2024.