Ethereum's next major upgrade has a test date. Developers agreed on October 6 for Glamsterdam to go live on the Sepolia testnet, the first public rehearsal before the upgrade heads toward the main network. The decision came out of the September 17 All Core Developers Consensus call, as both CoinCentral and Blockonomi reported, and it arrived with an unusual warning: the test itself could be easy to disrupt.

The warning came from consensus developer Potuz, who told the call that cheap test ether could let an attacker flood the network with fake block builders. "I can just spin up a thousand builders, rotate them, offer very high bids, and not produce payloads," Potuz said. "Any teenager can do this."

How the fake builder attack would work

Glamsterdam changes how Ethereum assembles blocks. The upgrade brings proposer-builder separation, known as ePBS, into the protocol itself through the EIP-7732 proposal. Under the design, specialized builders put together blocks of transactions and bid for the right to supply them, while validators accept the winning bid and expect the builder to deliver the transaction data.

On the real Ethereum network, competing in those auctions costs real money, which makes gaming the system expensive. On Sepolia, test ether is free. An attacker could create thousands of fake builder identities, win the auctions with worthless tokens, and then simply refuse to hand over the payloads. Blocks would arrive without the transaction data they were supposed to carry. As Potuz put it on the call, the cost of mounting the attack would be essentially zero, since test ether has no value.

That would not put real funds at risk. But it could freeze traffic on Sepolia and stop developers from properly evaluating how Glamsterdam performs under real conditions, which is the whole point of a public testnet. Existing safeguards may not be strong enough, since some client protections only activate after several payloads go missing. Developers said clients also need a way to ban individual builders so an attacker cannot return under a new identity and try again.

What Glamsterdam is built to do

Glamsterdam is Ethereum's next major upgrade, and it centers on capacity. The plan is to lift the block gas limit to around 200 million, up from 60 million in recent tests, so the network can fit far more activity into each block. For users, that could eventually mean lower fees during busy periods and room for more applications to run at once. The upgrade lands in a busy week for crypto: Bitcoin pushed past $80,000 on Friday, as covered on GenZ NewZ.

The upgrade also extends the window for sharing transaction payload data, from roughly two seconds to about nine seconds. The extra time gives nodes a better chance to process and spread the larger blocks Glamsterdam will produce.

A private rehearsal network called Devnet-11 completed a successful Glamsterdam transition on September 16. It ran 84,000 validators across multiple client implementations and raised the gas limit toward 200 million without losing finality, which gave developers the confidence to schedule the public Sepolia test.

The road from Sepolia to mainnet

Client teams now have until September 29 to release Sepolia-ready software. That leaves seven days before the October 6 fork, half the 14 days Ethereum usually sets aside for security review and bug testing. Developers accepted the shorter window because Sepolia uses a controlled validator set that is easier to recover if something goes wrong.

After Sepolia, a test on the Hoodi testnet is tentatively planned for October 27, though that date depends on how Sepolia goes. Ethereum has not set a mainnet activation date. The official roadmap still points to the fourth quarter of 2026, but no timestamp or epoch has been confirmed. According to CoinCentral, production builder software from the Titan and Ultrasound teams has not completed a Glamsterdam fork transition yet, another item on the checklist before the upgrade can ship.

Ethereum upgrades move through public testnets precisely so problems surface early, and the builder warning shows developers are taking that process seriously. If the September 29 software deadline holds and Sepolia forks cleanly on October 6, the upgrade stays on track for its Q4 target. If the fake builder problem shows up first, the rehearsal will have done its job.

Sources: CoinCentral.