Ethereum's Fusaka Upgrade Activates with Minor Prysm Client Bug Incident
Ethereum's Fusaka upgrade successfully activated on December 4, 2025, with zero downtime, enhancing data availability since the EIP-4844 implementation.
Hours after activation, a bug in the Prysm consensus client caused validation slowdowns and posed a risk to finalization. However, other consensus clients continued operating normally, preventing network disruption.
The Ethereum Foundation issued emergency guidance addressing the issue. The presence of ten other consensus clients demonstrated the critical value of client diversity.
The Prysm bug involved heavy historical state generation, potentially causing DoS-like conditions. A spike in stale attestations targeting checkpoint roots triggered historical state reconstructions.
Alternate clients such as Lighthouse, Teku, Nimbus, and Lodestar continued validating blocks while Prysm operators applied an emergency workaround by disabling last epoch targets.
Lido Finance reported minimal impact due to its distributed validator operations, with Prysm powering about 15% of the node operators.
Fusaka introduces PeerDAS data availability sampling, storing one-eighth of blob data, and the Blob Parameter Only (BPO) mechanism, enabling higher throughput. Blob base fees are now tied to execution costs and have risen 1,500 times to 1500 wei, with blob capacity targets increasing from 6 to 10 on December 9, 2025, and planned increases thereafter.
Layer-2 solutions like Optimism plan to integrate Fusaka features. Vitalik Buterin highlighted PeerDAS as a milestone comparable to sharding. Overall, the upgrade aims to improve data availability, throughput, and resilience across the Ethereum network.
