Ethereum has scheduled the Sepolia testnet fork for its next network upgrade, Glamsterdam, on Oct. 6, 2026, while a mainnet date has not been set.
Sepolia gets a date, mainnet remains blank
According to the Glamsterdam deployment table in Ethereum core developers’ coordination repository, Sepolia will activate at epoch 353024 and starting slot 11296768, which corresponds to Oct. 6, 2026 at 13:53:36 UTC.
In the same table, the epoch, slot, and timing fields for the Hoodi testnet and mainnet are all still listed as pending. The repository’s mainnet upgrade and incident response plan also remains an empty template, with the client coordinator and other roles not yet filled in.
The name Glamsterdam combines Gloas from the consensus layer and Amsterdam from the execution layer.
Developers flagged a testnet-specific economic risk
One of the mechanisms included in the upgrade is in-protocol proposer-builder separation, or ePBS, which lets block-building rights be assigned through bidding. The issue raised by developers is that the cost structure on mainnet and testnet is not the same.
According to CoinDesk’s report on the core developers’ meeting, consensus-layer developer Potuz described the attack surface this way: 「I can directly spin up 1,000 builders, bid in rotation, bid very high, and then not produce a payload.」 He added: 「Any teenager can do it.」
The scenario depends on testnet ETH having no real cost. On mainnet, participating in bidding carries an actual economic cost. On Sepolia, that constraint does not exist. That means an attacker could use free test ETH and multiple fake identities to repeatedly win block auctions and then submit nothing, causing the chain to stall.
Client teams face a Sept. 29 software deadline
The same CoinDesk report said developers need to deliver software releases that support the Sepolia fork by Sept. 29, leaving about seven days before the scheduled fork date.
No mainnet activation date has been assigned so far, and the roadmap still points to the fourth quarter of 2026. Ethereum’s usual process is to deploy on testnets first and then decide the mainnet schedule after reviewing the results, so the outcome of the Oct. 6 exercise will directly shape what comes next.