Ethereum developers have confirmed the Glamsterdam upgrade will deploy to the Sepolia public testnet on October 6, but they are warning that attackers using free test ether could stall the chain during testing by outbidding honest block builders and withholding the resulting blocks.
The date was accepted on the September 17 All Core Developers Consensus call, following a successful private devnet rehearsal. Devnet-11 completed its transition to the new Gloas block format and raised gas limits from 60 million to 200 million, clearing the way for the first public test. Hoodi, a second testnet with an open validator set, is expected to follow Sepolia, with its timing tied to how smoothly the Sepolia fork goes.
Ethereum’s public roadmap still lists Glamsterdam for the fourth quarter of 2026, but the mainnet date remains unconfirmed. The draft mainnet incident-response plan contains no activation epoch or timestamp yet, leaving those fields blank until client teams and coordinators are locked in.
Why fake builders are a problem
Glamsterdam’s headline change is enshrined proposer-builder separation, or ePBS, which formalizes the split between the parties who build blocks and the validators who propose them. Today that split runs through relays, trusted third-party middleware that has never been part of the protocol itself. On Sepolia, anyone can create a builder identity, and test ether is free. A researcher identified as Potuz argued on the call that this combination lets an attacker create disposable builder identities, win block bids with generous offers, and then simply refuse to release the execution payloads. The result is stalled or empty blocks on a testnet that application teams are counting on to be functional.
The concern is practical rather than theoretical. Sepolia uses a permissioned validator set controlled by client and testing teams, which gives developers some operational control. But application and infrastructure teams use public testnets to verify their software against real, producing blocks. A testnet that frequently stalls undermines that purpose and could force teams to retest against a moving target.
Client teams were urged to release Sepolia-ready software by September 29, leaving only seven review days before the fork. That is about half the usual review window, a compression several developers flagged as tight but acceptable given the devnet rehearsal went cleanly.
The performance case for Glamsterdam
The upgrade is designed to enable a major jump in capacity. Nethermind, one of the execution clients, ran a performance benchmark covering one of Glamsterdam’s biggest changes: block-level access lists, which allow transactions in a block to be processed in parallel rather than strictly in sequence. The client completed all 2,302 tests while processing 570.7 billion gas in three minutes and 15 seconds, an average of about 2.9 billion gas per second.
The upgrade also extends the window for spreading block data across the network from roughly two seconds to nine seconds, giving validators more time to receive and verify the larger blocks Glamsterdam is built to support. The 200 million gas limit tested on devnet remains a test setting, not a commitment for mainnet, but it signals the scale the upgrade is aiming at. For context, Ethereum’s mainnet has operated for years under a 30 million gas limit, with a recent gradual increase to 45 million.
CoinDesk reported the devnet rehearsal “continued confirming blocks” under the new block-building rules throughout the week, a clean run that gave developers confidence to set the public date. The benchmark measured Nethermind’s execution software specifically, not the speed of the full network, so real-world throughput will land lower, but the result confirms the core data structures can handle the load.
What Glamsterdam changes for users
For everyday users, Glamsterdam is not a fee-cutting upgrade on day one. Its effects arrive indirectly. Enshrined PBS removes a layer of off-protocol trust in relays, the intermediaries that currently connect builders and validators, a part of the stack that has been a recurring source of censorship and centralization concerns. Relays have at times filtered transactions based on sanctions lists, and their concentration among a handful of operators has drawn criticism from researchers who want that role embedded in the protocol with neutral rules.
The capacity gains come later, as the new block format allows gas limits to rise without the same network-fragility tradeoffs that limited previous increases. Faster block propagation and parallel execution are the mechanical foundations for that. The Ethereum Foundation has framed the upgrade as clearing the path for the next generation of scaling, following Fusaka’s focus on data availability for layer-2 networks. Wallets are also affected: CoinDesk has reported that Glamsterdam breaks the long-standing “21,000 gas” baseline that wallets rely on to estimate simple transfer costs, so wallet teams need to update their fee logic before mainnet.
The schedule from here
The sequence is Sepolia on October 6, then Hoodi, then a mainnet decision. If problems surface on Sepolia, the Hoodi date moves and the mainnet timeline slips with it. Developers have been explicit that the fourth-quarter target is aspirational until the two public testnets complete without major incidents.
The builder-abuse warning is a reminder that testnets are attack surfaces too. Sepolia’s permissioned validators limit the blast radius, but developers are expected to discuss additional safeguards for the Hoodi phase, where the open validator set makes the environment closer to mainnet conditions. How the October 6 fork handles misbehaving builders will inform whether ePBS needs extra protections before it reaches production.
For validators and node operators, the actionable item is the September 29 client release. Anyone staking on testnet infrastructure should plan to upgrade their Sepolia nodes promptly after that date. For application developers, the window between October 6 and the Hoodi fork is the time to test against real Gloas blocks. Forkcast, Ethereum’s live upgrade tracker, already lists the Sepolia fork as the next milestone for anyone following the schedule in real time.