Ethereum confirms Glamsterdam dates, but warns 'fake' builders could stall the chain

Ethereum developers have confirmed Oct. 6 as the date for the Sepolia public dress rehearsal of the Glamsterdam upgrade, but they cautioned that the testnet is vulnerable to manipulation. Because test ether on Sepolia carries no real cost, an actor can create numerous short-lived builder identities, outbid honest builders in block auctions, and then withhold the transaction payloads that validators expect. That behavior would produce blocks without transactions and could impede the infrastructure testing that the dress rehearsal is meant to exercise.

Developers emphasized that such an attack would not put mainnet funds at risk, since the activity would be confined to Sepolia. Still, the potential for repeated missing payloads raises operational concerns: current fallback mechanisms switch to locally built blocks only after several payloads are lost, and an attacker who can continually re-enter under new identities would be harder to stop. Consensus developer Potuz described the scenario in the core developer call, noting how easy it would be for someone to spin up many builders, submit very high bids, and then fail to produce payloads.

Client teams have been given until Sept. 29 to release Sepolia-compatible software, leaving just seven days for final reviews and bug-bounty testing — half the usual 14-day window the project typically allocates. Developers accepted the compressed timeline because Sepolia is more centralized and considered easier to recover if issues occur, but they also noted gaps remain: production builder software from teams Titan and Ultrasound has not yet completed a Glamsterdam fork transition.

Looking ahead, the next public test on the Hoodi network is tentatively scheduled for Oct. 27, with a decision on that date to follow Sepolia's results. A mainnet activation date for Glamsterdam has not been set. The upgrade itself aims to increase the gas capacity per block toward about 200 million and formally integrate the validator–builder relationship into protocol rules, changes that make robust dress rehearsals and protections against malicious builder behavior especially important.