Key Highlights
- Multi-party block construction (MPBC) is now live on Ethereum mainnet.
- Multiple builders can contribute transactions to a single block.
- The initial version uses an append-only model for additional block content.
Ethereum has deployed multi-party block construction (MPBC) on mainnet, introducing a system that allows more than one block builder to contribute transactions to a single block.
The Blockspace Forum described the deployment as an extension of Ethereum’s existing proposer-builder separation (PBS) architecture.
The initial version uses an append-only model. One builder creates the initial block, while additional builders can add eligible transactions before the block reaches the proposer.
How multi-party block construction works
Under Ethereum’s current PBS system, builders assemble blocks and compete to have their blocks selected by validators acting as proposers.
MPBC changes the construction process by allowing additional builders to contribute content without having to win the entire block auction.
The additional builders cannot modify transactions already included in the initial block. Their contributions are appended to the existing block.
This means MPBC changes how block contents can be assembled while leaving the existing proposer-selection process in place.
PBS auction remains unchanged
MPBC does not create a separate auction for validators.
Blocks containing contributions from multiple builders continue through Ethereum’s existing PBS infrastructure, where proposers select blocks through the established auction process.
Single-builder blocks also remain supported.
The change is therefore concentrated at the block-construction layer, rather than the mechanism through which validators select blocks.
Aestus, Titan and Ultra sound support the system
Several infrastructure providers are already participating in the MPBC rollout, including Aestus, Titan, and Ultra Sound.
The operators coordinate contributions from different builders before sending the resulting block through the existing transaction pipeline.
The Blockspace Forum has also published an open-source reference implementation for infrastructure providers interested in integrating MPBC.
The rollout requires coordination among builders, MPBC operators, relays and validators.
Blockspace competition and censorship
MPBC could change how transactions compete for inclusion in Ethereum blocks.
Under a conventional single-builder model, a transaction generally needs to be included by the builder that wins the block auction.
With MPBC, another builder can potentially contribute transactions without controlling the entire block.
The Blockspace Forum has also connected the design with censorship-resistance concerns, arguing that content excluded by one builder could potentially be added by another participating builder.
The practical effect will depend on how many builders participate and how the system is used across Ethereum’s block-building infrastructure.
More than 90% of connected validators supported the rollout
The Blockspace Forum said more than 85% of the transaction pipeline and over 90% of connected validators supported the initial deployment.
These figures refer to infrastructure connected to the MPBC rollout. They should not be interpreted as meaning that more than 90% of all Ethereum validators are currently running MPBC.
The Forum listed participants include Titan Builder, Ultra Sound, Bomba, Quasar, ETH Gas, NasdaqBTCS, Kraken, Kiln, Blockdaemon, Lido, Puffer, and SSV Network.
Ethereum users do not need to change anything
MPBC operates at the infrastructure layer used to construct and relay blocks.
Wallets and decentralized applications can continue submitting transactions through existing Ethereum infrastructure. Layer 2 networks are also not required to change how they submit transactions because of the deployment.
The changes are primarily relevant to block builders, relays, MPBC operators, and other infrastructure providers involved in Ethereum’s block-production pipeline.
Separate from Other Ethereum infrastructure projects
The MPBC rollout comes alongside other infrastructure developments across the Ethereum ecosystem, although they address different parts of the network.
On September 1, Lido funded the ValOS framework, which focuses on validator operations such as key management, security, and incident response.
The Aave community has also approved plans for Aave V4 on Ethereum mainnet, targeting lending infrastructure rather than block construction.
These projects are separate from MPBC. ValOS addresses validator operations, Aave V4 concerns DeFi infrastructure, while MPBC focuses on assembling block contents.
What comes next
The September 16 deployment is the first production implementation of MPBC on Ethereum.
The Blockspace Forum said further specifications, documentation, and live data will be released as development continues.
The system is also being developed with compatibility in mind for Ethereum’s work on enshrined proposer-builder separation (ePBS) and the proposed FOCIL censorship-resistance mechanism.
For now, MPBC adds another layer to Ethereum’s block-building process without changing how users transact on the network. Its longer-term effect will depend on participation from builders and infrastructure providers and how the system performs under real network conditions.
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