Multi-party block construction (MPBC) went live on the Ethereum mainnet on September 16, letting several builders contribute transactions to a single block. Early data show fuller blocks and faster transaction inclusion, without any change to the underlying protocol.
Multiple builders can now assemble a single Ethereum block together, replacing the one-builder-per-block model that has defined the network since the Merge. As of September 16, multi-party block construction started running on Ethereum mainnet, allowing several builders to add transactions to the same block without any protocol-level changes.
What multi-party construction changes
Since the 2022 Merge, Ethereum has used proposer-builder separation: validators propose blocks, but specialized builders assemble them by competing in an auction, and the winner's block gets included. MPBC alters that model so multiple builders can each contribute transactions to one block, creating several inclusion paths instead of funneling everything through a single winner.
The early results are notable. Some multi-party blocks have included up to 810 additional transactions compared to a single-builder block. In certain cases, gas usage jumped nearly 14-fold. In the first 24 hours, roughly 3% of Ethereum blocks were built using the multi-party approach. Since testing began in late August, more than 20,000 transactions have been included faster.
How the rollout came together
The Blockspace Forum coordinated the initiative, running workshops through 2026 that culminated in a Cannes meeting bringing together 32 teams, which together account for over 95% of out-of-protocol block contributions on Ethereum. Foundational research published in May 2026 set the design principle that MPBC had to be strictly additive, with no hard forks or consensus-level modifications. The system also does not depend on relays for its core function, which reduces the chokepoint risk.
Alongside the launch, the team released public documentation and a live dashboard tracking participation and transaction-inclusion metrics.
Why builder centralization matters
Under the current model, whichever builder wins an auction controls transaction ordering within that block, which can let it prioritize its own profitable trades, sandwich trades, or front-run and exclude others. MPBC does not eliminate these dynamics, but it dilutes them: when multiple builders contribute to the same block, no single entity holds full control over ordering or inclusion, giving other transactions additional pathways into blocks.
Source: Crypto Briefing
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