Ethereum brings Multi-Party Block Construction to mainnet
Ethereum has begun using Multi-Party Block Construction (MPBC) on mainnet, introducing a new approach that allows multiple block builders to contribute transactions to the same block.
The system was developed through the Blockspace Forum, a group focused on improving how Ethereum transactions move through the network's block-building infrastructure. The forum's published design describes MPBC as an extension of Proposer-Builder Separation (PBS) rather than a replacement for it. Under the existing system, specialized builders construct blocks and compete to have their block selected by a validator proposer. MPBC adds another path by allowing contributions from other builders to be appended to the winning block.
The initial design is strictly additive. A base block is not rewritten; instead, eligible transactions that were absent from that block can be added afterward. The Blockspace Forum says the approach is intended to improve transaction inclusion, blockspace utilization and the robustness of Ethereum's transaction pipeline.
How multi-party block construction works
Under conventional PBS, one winning builder constructs the block that ultimately reaches the proposer. This can leave some transactions outside the winning builder's view or distribution network, potentially causing them to wait for a later block.
MPBC changes this by creating additional inclusion paths. According to the Blockspace Forum's technical design, operators receive blocks and transaction information from multiple builders. They start with the highest-bid single-party block and can append eligible, non-conflicting transactions from contributing builders. The resulting multi-party block is then compared with the highest-value single-party block available at delivery time.
The proposer still makes the final choice. An MPBC block is delivered only when it offers at least as much value as the relevant single-party alternative. This means the mechanism extends the existing auction rather than removing the economic competition between builders.
The first version is deliberately limited in scope. The design initially focuses on transactions that can be added without changing the execution guarantees of the base block, while future versions could support additional transaction types and services.
Multiple builders can contribute to one Ethereum block
The main change introduced by MPBC is the ability for a builder to earn from transactions it contributes even if it does not win the entire block-building auction.
The Blockspace Forum's research says the system can broaden access to Ethereum blockspace because transactions from several builders can find their way into the same block. Smaller or specialized builders therefore do not necessarily need to win an entire slot to contribute value.
This also changes the economics of block construction. In a single-party model, builders primarily compete to win the whole slot. Under MPBC, a builder can contribute eligible transactions to another builder's base block and potentially receive compensation for the value it adds.
The design also creates a role for operators, which combine eligible contributions and deliver the resulting block to the proposer. The exact rules for distributing the additional value generated by MPBC remain an area for further development, according to the Blockspace Forum.
Why Ethereum's blockspace pipeline is changing
Ethereum produces a block roughly every 12 seconds, making the efficient allocation of blockspace important for users, builders and validators.
The Blockspace Forum has identified several structural issues with the existing transaction pipeline, including concentration among builders, unpredictable transaction inclusion and limited opportunities for different participants to contribute to blocks. Its research argues that multiple inclusion paths could help address some of these limitations.
The forum's Cannes 2026 workshop explored block merging and multi-party construction as ways to allow transactions from multiple builders to reach the same block. It said a testnet pilot was being extended toward mainnet deployment, with the broader design intended to work alongside developments such as ePBS and FOCIL.
MPBC does not require an Ethereum consensus-layer change in its initial design. Instead, the mechanism is implemented through infrastructure operated by builders and operators around the existing block-production process.
Potential impact on transaction inclusion
For Ethereum users, one of the most important potential effects is improved access to blockspace.
When a transaction is known to one builder but not another, a single-builder pipeline can leave that transaction waiting if the builder with the transaction does not win the slot. MPBC provides an additional route for eligible transactions to be included through another participating builder.
The Blockspace Forum says this can make transaction inclusion more predictable and reduce the need for users to overbid simply to compensate for uncertainty. However, these are design objectives rather than guarantees for every transaction, and the initial implementation has restrictions on which transactions can be merged.
The system may also have implications for censorship resistance. With multiple builders able to contribute to a block, transaction inclusion is less dependent on the transaction view of a single winning builder. The forum identifies this as one of the potential benefits of a more distributed block-construction process.
MPBC is designed to work with future Ethereum upgrades
The Blockspace Forum's MPBC design is intended to coexist with other changes to Ethereum's transaction pipeline.
The forum specifically describes compatibility with FOCIL, or EIP-7805, and future enshrined proposer-builder separation (ePBS). These technologies address different parts of Ethereum's block-production architecture, while MPBC focuses on creating multiple paths for block contributions.
The project can also potentially be expanded through sub-slots, where multiple builder contributions could occur more frequently within Ethereum's approximately 12-second slot. The Blockspace Forum has presented sub-slot auctions as a future direction rather than part of the initial MPBC implementation.
Future work includes questions around value distribution, trust assumptions, broader transaction coverage, proposer services and networking performance.
Ethereum's block-building infrastructure enters a new phase
The launch of Multi-Party Block Construction represents a change in how Ethereum's off-chain block-building infrastructure can coordinate multiple participants.
Instead of requiring one builder to supply all of the useful transactions in a block, MPBC allows a base block to be supplemented by eligible contributions from other builders. The proposer still evaluates the resulting block through the existing economic selection process.
For now, the system remains an initial implementation with defined limits. But the architecture provides a framework for additional inclusion paths while remaining compatible with Ethereum's broader blockspace research and planned infrastructure developments.
Other crypto developments
Alongside Ethereum's MPBC deployment, several other developments featured in the source update.
The Federal Reserve raised its federal funds target range by 25 basis points to 3.75%-4% on Sept. 16, 2026. The FOMC approved the decision unanimously by a 12-0 vote. The central bank said inflation remains elevated and that the action was intended to support a return toward its 2% inflation goal.
The Fed's September projections also provide updated views from individual FOMC participants on the appropriate path for interest rates. The projections are assessments of appropriate policy rather than forecasts of guaranteed future rate decisions.
In DeFi, Aave continues to develop V4, its next-generation protocol architecture. Aave's published materials describe V4 as introducing a Hub-and-Spoke architecture, risk premiums and a redesigned liquidation engine, while also supporting additional real-world-asset and institutional lending use cases.
Other developments cited in the original roundup include a reported Yam Finance governance attack, Hyperliquid's introduction of Share, Oasis Pro Markets joining DTCC's Fund/SERV, and Arc going live on mainnet. These developments are separate from Ethereum's MPBC launch and are therefore treated as additional market updates rather than part of the main Ethereum story.
