Developers in the Ethereum ecosystem have announced that the mainnet activation for the highly anticipated Fusaka upgrade is scheduled for December 3, 2025. This marks a significant acceleration from earlier estimates, which anticipated a rollout extending well into 2026. The adjustment in the timeline is prompted by insights gained during recent testing on the Fusaka Devnet-5, where a series of software bugs and setup errors cropped up, limiting the time available for developers to measure data capacity accurately.
During a brief interval of stability on the test network, developers reached a consensus to implement a phased increase in blob capacity. The initial adjustment will raise the capacity to 10/15 blobs per block, followed by a further expansion to 14/21 blobs. In the context of Ethereum, a “blob” is characterized as a temporary data packet utilized by rollups—scaling solutions that help post transaction data to the blockchain at a reduced cost.
To further validate these changes, a new test network, dubbed Devnet-6, is set to launch soon. This network will help confirm the new capacity numbers prior to their deployment on public testnets and, ultimately, the mainnet.
However, challenges remain. Developers observed that the Prysm client, one of the main validator clients within Ethereum’s proof-of-stake framework, experienced difficulties under high load conditions, resulting in orphaned blocks. An orphaned block is one that was successfully produced by a validator but subsequently rejected by the network in favor of a different block in the same position on the blockchain.
Ethereum researcher Justin Traglia noted that a bug in the ckzg library, which is critical for blob verification, has been resolved. In addition, efforts are underway to prepare a lighter version of the library, which aims to facilitate easier upgrades for client teams.
The expedited timeline for the Fusaka upgrade is strategically designed to provide additional blob space to accommodate increasing transaction volumes and keep fees low. Developers are committed to incrementally raising capacity to mitigate any risks associated with overwhelming the network. In discussions, it was also confirmed that client teams would collaborate to issue bundled releases for the upcoming October testnet updates and conduct shadow forks—essentially copies of Ethereum’s main network—prior to deployment. Nonetheless, developers acknowledged the possibility of delays should new bugs arise or if testing outcomes do not meet expectations.
Fusaka follows the earlier Pectra upgrade, which was introduced in May and aimed to enhance the user experience on the Ethereum network by increasing staking limits and boosting data capacity to alleviate costs associated with rollups. While Pectra was heralded as a pivotal development in Ethereum’s roadmap, experts cautioned that ongoing scaling pressures would continue to challenge the network, with rollups requiring significantly more blob capacity to operate efficiently as usage escalates. The Fusaka upgrade is positioned to address these burgeoning demands, implementing increased blob limits more swiftly in response to rising network activity.
