Ethereum core developers confirmed the Glamsterdam upgrade will activate on the Sepolia testnet on October 6, following a successful private rehearsal that tested higher block capacity. The decision came during a September 17 All Core Developers Consensus call, where teams also raised concerns that inexpensive test ether could enable malicious actors to disrupt the public testing phase through builder abuse.

The upgrade centers on EIP-7732, known as Enshrined Proposer-Builder Separation, or ePBS. This change moves the assembly of transaction payloads into the protocol itself, separating it from validators’ consensus duties. Builders submit bids for the right to supply an execution payload. Once a proposer accepts the winning bid, the builder is expected to release the corresponding transactions.

Risk of Disposable Builders on Public Testnets

Consensus developer Potuz warned that the economics differ sharply on a public testnet. Because test ether carries no meaningful cost, an attacker could create numerous disposable builder identities, submit aggressively high bids, and then withhold the promised payloads after winning.

He added that the barrier is low enough that nearly anyone could attempt it. The concern is framed as a testnet availability issue rather than a threat to mainnet funds. On mainnet, the cost of obtaining real ether limits empty-block or withholding behavior. On Sepolia, persistent disruption becomes far cheaper.

Existing client circuit breakers may not fully address the problem. Some fall back to locally built blocks only after multiple missed payloads, and developers noted a lack of universal mechanisms to identify and restrict individual abusive builders that rotate identities.

Sepolia’s permissioned validator set, controlled by client and testing teams, provides more operational control than fully open networks. Hoodi, which features an open validator set for staking and protocol testing, is expected to follow. Tentative timing discussed for Hoodi points toward late October, contingent on Sepolia’s progress.

Devnet-11 Completes Capacity Rehearsal

The confirmation of the October 6 date followed the successful completion of Glamsterdam Devnet-11. That controlled network was designed as a “happy-path” rehearsal rather than an adversarial environment. Genesis occurred around September 14, with the Gloas transition on September 16.

Organizers raised the block gas limit from 60 million to 200 million shortly afterward. The network ran with approximately 84,000 validators across multiple clients and included the core EIP set planned for Glamsterdam. It maintained finality throughout the transition. The 200 million gas parameter remains a test setting and is not a confirmed mainnet target.

EIP-7732 also extends the execution-payload propagation window from roughly two seconds to around nine seconds, giving nodes additional time to distribute and validate larger payloads. The upgrade further incorporates Block-Level Access Lists and gas-pricing adjustments intended to support higher Layer 1 throughput.

Compressed Client Review Window

Client teams face a tightened schedule. Developers discussed a September 29 target for release-ready software, leaving only seven days before the Sepolia activation. Ethereum’s standard process typically calls for at least 14 days of review, covering internal security checks, bug-bounty exposure and possible external audits.

Participants accepted the shorter window in part because Sepolia’s relatively centralized validator set allows easier recovery if software issues arise. Core developer Alex Stokes encouraged teams to publish earlier where possible to maximize review time. Once available, the software can enter the bug-bounty process immediately.

Earlier testing rounds encountered setbacks, including non-finality events on previous devnets and performance issues affecting specific clients. Devnet-11 was structured to avoid deliberate attacks, reserving adversarial scenarios for longer-running environments.

Mainnet Timing Remains Open

Ethereum’s public roadmap continues to list Glamsterdam for the fourth quarter of 2026, but no mainnet activation epoch or timestamp has been set. The draft mainnet incident-response plan leaves those fields blank pending further multi-client testing and coordination.

Successful public testnet performance on Sepolia and subsequent stages remains a prerequisite before developers finalize mainnet timing. Application and infrastructure teams that rely on testnets for software validation have been advised of the potential for temporary disruption during the initial public phase under the new builder rules.

Client teams now focus on the September 29 software deadline ahead of the October 6 Sepolia fork. Further milestones, including Hoodi, will depend on the outcome of that first public deployment.