Ethereum core developers have finalized the timeline for the Fusaka upgrade, with mainnet activation scheduled for December 3, 2025. The hard fork will implement approximately 11-12 Ethereum Improvement Proposals (EIPs) focused on backend protocol improvements, including scalability enhancements, node efficiency, and data availability upgrades.
A key feature of Fusaka is the phased expansion of blob capacity through Blob Parameter Only (BPO) forks. The first BPO fork, scheduled for December 17, will increase blob capacity from the current 6/9 (target/max) to 10/15 blobs. A second BPO fork on January 7, 2026, will further expand limits to 14/21 blobs, more than doubling total capacity within two weeks of mainnet activation.
The upgrade includes implementation of PeerDAS through EIP-7594, allowing validator nodes to verify data by sampling small pieces rather than downloading entire blobs. This reduces bandwidth and storage requirements while enhancing Layer 2 rollup scalability. Additional improvements include bounded base fees for blob transactions via EIP-7918, creating more predictable transaction costs for data-heavy applications.
Testing will follow a conservative four-phase approach across Ethereum testnets: Holesky upgrade on October 1, Sepolia on October 14, and Hoodi on October 28. Node operators face release deadlines ranging from September 25 for Holesky to November 3 for mainnet preparation.
The upgrade addresses mounting scalability pressures as blob usage has increased significantly since the Dencun upgrade, with average blob count per block currently at 5.1 compared to 0.9 in March 2023. The Ethereum Foundation has announced a four-week code audit program with $2 million in rewards for developers who discover vulnerabilities in the Fusaka codebase.
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