Bitcoin Core Integrates Cluster Mempool to Improve Transaction Processing and Layer-2 Security

Bitcoin Core has implemented Cluster Mempool, a comprehensive restructuring of how nodes organize pending transactions, following its merge into the codebase via pull request #33629 on November 25, 2025^[1].

Developed by Suhas Daftuar and Pieter Wuille, the new architecture addresses fundamental misalignments between how nodes previously selected transactions for mining versus mempool eviction. Prior to version 30.0, Bitcoin Core used separate rankings that could produce contradictory results, creating uncertainty for transaction propagation and second-layer protocol security^[1].

The redesign introduces two key organizational concepts: clusters and chunks. Clusters group related unconfirmed transactions together, while chunks represent smaller transaction sets sorted by feerate within each cluster. This structure enables miners to efficiently construct optimal block templates by selecting the highest-feerate chunks across all clusters^[1].

The architecture simplifies transaction replacement logic and establishes a unified 64-transaction, 101 kilovirtual-byte limit per cluster, replacing previous ancestor-based restrictions. By maintaining relatively small clusters, the system reduces computational overhead while ensuring predictable behavior for fee-bumping and layer-2 enforcement transactions^[1].

The improvements aim to support mining decentralization by ensuring all miners access the most profitable transaction sets, while providing users with more reliable fee estimation and confirmation times^[1].