Ethereum developers have set December 3, 2025, as the mainnet activation date for the Fusaka upgrade, advancing a timeline previously expected to extend into 2026.
The decision follows testing on the Fusaka Devnet-5 network, which revealed software bugs and setup errors, shortening the window for data capacity analysis.
Impact of Fusaka on the Ethereum ecosystem
The Fusaka upgrade is designed to benefit all participants in the network.
Users can expect faster transaction confirmations during peak periods, with gas fees becoming more predictable, though still subject to network demand. Developers will gain larger blob capacity and PeerDAS support, enabling rollups and other Layer 2 solutions to submit data more reliably and efficiently.
They agreed to raise Ethereum’s blob capacity in two steps: initially to 10/15 blobs per block, then to 14/21.
Blobs are a relatively new data structure introduced through EIP-4844, also called Proto-Danksharding.
They act as temporary data packets that rollups use to post transaction data more cheaply. Unlike traditional transactions, blobs aren’t stored permanently on the blockchain and remain accessible for only about 18 days, making them a cost-efficient way to scale Ethereum while easing network congestion.
Testing highlights and client updates
Developers identified performance issues during testing, particularly with the Prysm client, one of Ethereum’s major validator clients, which struggled under high loads, resulting in orphaned blocks. Validator clients are responsible for proposing and verifying blocks in Ethereum’s proof-of-stake system, and orphaned blocks occur when the network rejects a block despite it being produced correctly.
Ethereum researcher Justin Traglia confirmed that a bug in the ckzg library, used for blob verification, had been fixed. A lighter version of the library is being prepared to simplify upgrades for client teams.
Another test network, Devnet-6, is expected to launch soon to validate the new blob capacity numbers before they appear on public testnets and the mainnet.
Fusaka follows Pectra
The Fusaka upgrade follows Pectra, rolled out in May 2025, which increased staking limits, improved usability, and expanded data capacity to reduce costs for rollups.
Previously Pectra was described as a turning point for Ethereum’s roadmap but cautioned that scaling pressures would persist, particularly as rollups demand more blob capacity to stay uncongested.
Fusaka is designed to address that challenge by phasing in higher limits earlier to match growing network activity.
Preparation and risk management
According to meeting notes from researcher Christine Kim, the accelerated timeline aims to give rollups sufficient blob space to handle transactions and maintain low fees.
Developers plan to activate the Holesky testnet on October 1, followed by Sepolia on October 14 and Hoodi on October 28.
The mainnet target date is set for December 3, 2025, although these dates may shift depending on testnet performance and the need to address any issues that arise.
Looking back and ahead
Ethereum has recently experienced notable market activity as the network prepares for Fusaka. Prices dipped below $4,000, triggering liquidations of leveraged positions and highlighting the volatility that often accompanies major network developments.
At the same time, Ethereum has surpassed Bitcoin in spot trading volume on major exchanges.
With layered testing, incremental upgrades, and active participation from validator client teams, the upgrade represents a measured approach to one of Ethereum’s most significant scaling milestones yet.
Alex Shepel
Ethereum Fees Hit $100 Post BlackRock's ETH ETF Filing
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