Ethereum's Sepolia testnet will activate the Glamsterdam upgrade on October 6, 2026, giving developers the first fixed date to test the update's scaling changes. Developers have warned that free Sepolia ETH and disposable builder identities could let an attacker repeatedly disrupt Glamsterdam's block-building auctions, though the risk stays confined to the testnet and does not threaten mainnet funds.
Sepolia activation set for October 6
Ethereum's Sepolia testnet is scheduled to activate Glamsterdam on October 6, 2026, at 13:53:36 UTC, at epoch 353024 and slot 11296768, through a proposal merged into Ethereum's protocol-management process. The date places Sepolia at the front of Glamsterdam's expected rollout. Ethereum's official roadmap still lists the upgrade as in testing and targets mainnet deployment in Q4 2026.
Glamsterdam centers on scaling changes, including enshrined proposer-builder separation, or ePBS, which changes how the network handles the relationship between validators proposing blocks and builders assembling their execution payloads. Sepolia is therefore more than a date-setting exercise: it is where developers can examine how the new design behaves before any decision on moving further through the rollout.
Free Sepolia ETH could let fake builders repeat auction wins
CoinDesk reported that free Sepolia ETH and disposable builder identities could allow an attacker to repeatedly win block-building auctions and then withhold promised transaction payloads. A participant could return under a new identity after disrupting an auction, and low-cost testnet ETH removes the financial friction of repeated bids.
The reported scenario concerns interference with testing rather than value extraction, and the risk stays limited to Sepolia. It does not threaten mainnet funds, although persistent withheld payloads could stall or complicate testing of Glamsterdam's auction and delivery mechanics.
Payload delivery becomes the test for ePBS
Glamsterdam's ePBS design, outlined in EIP-7732, has builders commit to execution payloads for later reveal rather than deliver them at the initial commitment, with payload-timeliness committees that would monitor whether winning builders complete delivery. Sepolia will test that arrangement under conditions where free testnet ETH and disposable identities could support repeated wins followed by withheld payloads.
Mainnet timing stays undecided
The September 17 All Core Developers call recorded October 6 for Sepolia and tentatively discussed October 27 for Hoodi, but the discussion did not set a mainnet fork date. Sepolia's October 6 activation remains the next confirmed milestone in testing Glamsterdam's scaling work and payload-delivery safeguards.
Source: Crypto Daily
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