Ethereum’s Glamsterdam Test Puts Builder Identity Under Pressure

Ethereum’s Glamsterdam upgrade is moving toward its public Sepolia test on October 6, 2026, but the most important question may be whether the network can identify and contain unreliable block builders before larger blocks become a production reality.

CoinDesk reported on September 17 that private rehearsals for Glamsterdam had demonstrated larger blocks, block-level access lists and a route toward substantially higher capacity. Those changes are intended to let Ethereum process more activity in each block and make access to state data more predictable for validators and clients.

The public Sepolia rehearsal will put a different part of that design under pressure. Developers have warned that attackers could use free test ether and disposable builder identities to repeatedly win block auctions, then withhold the transaction payloads needed to complete those blocks. Such an attack would not place mainnet funds at risk, but it could interrupt the test, expose weaknesses in Ethereum’s fallback procedures and complicate the upgrade timetable.

That makes Sepolia more than a performance demonstration. It will also be a test of identity, accountability and coordination in a system where the party proposing a block may not be the same party that assembled its transactions.

Why builders matter to Glamsterdam

Ethereum’s block production system separates several responsibilities. Validators remain responsible for proposing blocks, while specialized builders can assemble transaction packages and compete to have those packages included. Relays and related infrastructure can help transmit or verify the resulting block data before it reaches the validator responsible for proposing it.

This separation has allowed block construction to become more specialized. Builders can invest in sophisticated software, transaction ordering strategies and infrastructure that ordinary validators may not operate themselves. In return, they compete in an auction environment in which the validator generally prefers the block that offers the highest value.

That model creates efficiency, but it also creates a point of dependence. A builder can win an auction and secure a place in the production process while failing to deliver usable data at the expected time. If the system treats the builder’s winning bid as sufficient evidence of reliability, an attacker may be able to cause repeated delays without needing to control a large share of Ethereum’s staking power.

The proposed attack is especially relevant on Sepolia because test ether has no market value and can be distributed freely. An adversary does not need to risk substantial capital to participate in auctions. If builder registration or reputation is easy to reset, the attacker can create new identities after each failed or abusive attempt. The result would be a form of operational disruption based less on cryptographic power than on the ability to exploit weak identity and fallback rules.

CoinDesk said in a follow-up that fake builders could stall the chain during the public rehearsal. The warning focuses attention on whether Ethereum can distinguish a legitimate new participant from an identity created solely to interfere with block production.

The threat is limited, but the lesson is not

A Sepolia disruption would not freeze Ethereum mainnet or endanger users’ assets. Test networks exist precisely so developers can find problems in conditions that are safer and less costly than production. Yet the absence of direct financial damage does not make the exercise irrelevant.

A successful attack could reveal that a capacity upgrade works only when all participants behave honestly. That is a meaningful limitation for a public blockchain. Ethereum must assume that some actors will be opportunistic, anonymous or deliberately hostile. Its safety mechanisms therefore need to function even when a builder has no long term reputation and little to lose.

The immediate technical question is whether validators and clients can fall back to another block production path quickly enough when a builder fails. A robust fallback should prevent one unresponsive or deceptive participant from holding up the chain. It should also avoid creating incentives for validators to accept incomplete data merely to preserve block times.

There is a tradeoff. Aggressive rejection rules may protect the chain from withheld payloads, but they can also discard profitable blocks, reduce participation or increase the operational burden on validators. Loose rules may preserve short term efficiency while allowing unreliable builders to win repeatedly. Glamsterdam’s test will help show where that balance currently sits.

The identity issue adds another layer. In many digital markets, repeated misconduct can be deterred through deposits, legal accountability or exclusion from future participation. A permissionless blockchain cannot rely on all of those tools. If a builder can simply abandon one identity and return under another, reputation systems have limited value. Developers may need stronger economic costs, clearer eligibility checks or technical methods that allow participants to be evaluated without turning block building into a closed club.

Capacity brings verification costs

The case for Glamsterdam is not limited to faster transactions. Larger blocks can expand the amount of activity Ethereum settles, potentially improving the economics of decentralized applications, payments and other forms of onchain use. Block-level access lists are intended to make state access more explicit and predictable, which can help clients prepare for the work required to execute and verify blocks.

But higher capacity can shift costs rather than eliminate them. A block that contains more transactions may require more bandwidth, storage and computation from validators. If professional operators can handle those demands more easily than ordinary participants, the network could become more dependent on large infrastructure providers.

That concern is central to Ethereum’s long running debate over scalability. More blockspace can reduce congestion and transaction costs for users, but it can also raise the minimum hardware and network quality needed to verify the chain. If fewer participants can keep up, the system may become more concentrated even if its validator count remains high.

Access lists and related changes are therefore important not only because they can increase capacity, but because they may make the cost of processing that capacity more predictable. Predictability helps client teams optimize software and helps operators assess whether they can continue validating from their own infrastructure. The test must establish whether those gains are real under adverse conditions, not just in controlled benchmarks.

The builder attack fits directly into this question. If larger blocks depend on a complex chain of builders, relays and fallback mechanisms, then capacity may be purchased with greater operational fragility. The upgrade will be stronger if it demonstrates both higher throughput and a reliable response when an intermediary misbehaves.

A compressed window for review

The Glamsterdam process also places pressure on the developers responsible for Ethereum’s execution and consensus clients. Client teams need time to implement the changes, test interactions between software versions and investigate failures that appear only under realistic network conditions. A public test that produces ambiguous results can force difficult decisions about whether to delay, narrow or redesign parts of an upgrade.

The shortened review window makes Sepolia’s data especially important. Developers will need to distinguish between ordinary testnet instability and evidence of a structural weakness. A temporary interruption caused by an isolated configuration error may be manageable. Repeated auction capture by disposable identities would point to a problem in the assumptions behind the builder workflow.

Useful results will include more than a simple measure of transactions per second. Teams should examine how often blocks are missed, how quickly validators use fallback paths, whether incomplete payloads propagate across clients and whether different client implementations respond consistently. They will also need to monitor the distribution of successful bids. If one actor or a rotating group of identities can dominate the auction with little capital, the test may reveal an economic vulnerability even if the chain continues producing blocks.

Client diversity matters as well. A fallback mechanism that works on one implementation but fails on another could create uneven behavior across the validator set. That would make incident response harder and could increase the risk of a chain split or prolonged periods of degraded service. Public rehearsals are valuable partly because they expose these differences before an upgrade affects assets with real value.

The broader infrastructure question

For institutional users, the outcome will matter beyond Ethereum’s developer community. Banks, payment companies, asset managers and regulated crypto businesses increasingly evaluate public blockchains as settlement infrastructure. Their concerns include transaction capacity, predictable costs, operational resilience and the ability to explain who controls critical parts of the process.

A builder market that is efficient but difficult to audit may raise questions for firms with governance and risk obligations. Institutions do not necessarily require every participant to be identified by name, but they do need confidence that disruptions can be contained and that critical dependencies are understood. Repeated block failures caused by anonymous, disposable operators would complicate those assessments.

Regulators in different jurisdictions are also examining how responsibility is distributed across crypto infrastructure. Rules may distinguish between validators, trading platforms, custodians and software providers, but technical systems do not always fit neatly into legal categories. A builder that influences transaction ordering and block inclusion may perform an economically important function without looking like a conventional intermediary.

Ethereum’s response could shape that discussion. If the network relies on open participation while maintaining effective safeguards against abuse, it offers a stronger case for decentralized infrastructure that can support institutional activity. If higher capacity requires informal gatekeepers or a small set of trusted operators, the system may become easier to manage but less meaningfully open.

What success should look like

A successful Sepolia test should therefore meet several standards at once. It should show that larger blocks can be propagated, executed and verified without excluding too many ordinary validators. It should demonstrate that access lists provide useful predictability under real network conditions. It should also show that a builder can fail, misbehave or disappear without stopping the chain.

The strongest result would not be a test with no disruption. A controlled attack that is detected, contained and documented could provide more valuable evidence than a flawless rehearsal. Developers need to know which signals identify a malicious builder, how quickly fallback activates and what costs the network pays when it rejects a winning bid.

The test should also produce clear guidance for mainnet operators. Validators need to understand which software versions are compatible, which settings affect fallback behavior and how to respond if builder infrastructure becomes unreliable. Without that operational clarity, even a technically sound upgrade could create uneven preparedness across the network.

Glamsterdam’s public rehearsal is consequently a measure of Ethereum’s institutional maturity as much as its technical ambition. The upgrade promises more blockspace, but the harder task is preserving the properties that make that blockspace credible: independent verification, broad participation and resistance to manipulation.

If Sepolia withstands the builder identity challenge while delivering the expected capacity gains, Ethereum will have evidence that its scaling strategy can handle both greater demand and adversarial behavior. If fake builders repeatedly stall the network, the failure will still be useful, provided developers treat it as a design signal rather than an inconvenience to be worked around.

The central question is not whether Ethereum can make blocks bigger. It is whether the network can make them bigger without giving unreliable or unaccountable intermediaries too much influence over who gets to use its limited space. Glamsterdam’s answer will begin taking shape on October 6.