Glamsterdam Upgrade Set for Oct. 6 Debut on Ethereum's Sepolia Testnet

Ethereum developers have penciled in Oct. 6, 2026, at 13:53 UTC as the target time for activating the Glamsterdam upgrade on the Sepolia testnet, though the date isn't locked in yet, since another round of private devnet testing still has to clear before the public testnet fork can move forward.

Why the Sepolia Date Is Still Conditional

Notes from ACDC #186 and follow-up reporting from Ethereum protocol researcher Christine D. Kim confirm that this timeline is far from set in stone. When developers picked the Sepolia date, they hadn't yet achieved a fully stable Glamsterdam run on any private development network.

Since then, the testing roadmap has already shifted once. In a Sept. 11 update, Kim noted that the team's focus has moved to Glamsterdam-Devnet-11, slated to go live Monday, Sept. 14. That's a change from earlier plans, which had pointed to Devnet-10 as the next major checkpoint.

No Confirmed Dates Yet for Hoodi or Mainnet

Neither the Hoodi testnet nor Ethereum's mainnet has a locked activation date at this point. There's been talk among developers of a possible December mainnet launch, but whether that timeline holds up will come down entirely to how the remaining tests perform.

How the Sepolia Epoch Was Chosen

At the Sept. 3 All Core Developers Consensus call, the team landed on Sepolia epoch 351232 as the proposed activation point, which Kim calculated corresponds to Oct. 6 at 13:53 UTC. Notably, this decision was made before the private Glamsterdam test networks had shown reliably stable results.

Settling on a specific epoch gives client teams, infrastructure providers and app developers a shared target to plan around, but it's not a done deal. If the next testing round turns up a serious issue, or if client teams can't get dependable releases ready in time, the fork could still get pushed back.

Devnet-9's Finality Issues Keep the Caveat Alive

That word of caution matters even more given that Devnet-9 ran into finality trouble. Meeting documentation shows the network ran with roughly 1,000 validator nodes, the largest validator count of any Glamsterdam devnet tested so far.

For a chain to reach finality, enough validators need to agree on its current state. When that fails on a test network, developers have to dig in and figure out whether the root cause is client software, validator participation levels, network setup, or some interaction between separate protocol changes running at once.

Testing Sequence Has Already Moved Past the Original Plan

Originally, Devnet-10 was slated as the next step after issues surfaced in earlier rounds. But Kim's newest update points to Devnet-11 as the test developers are now watching most closely, a sign that the private testing sequence has already progressed further than initially expected.

A clean, stable Devnet-11 run would hand Ethereum client teams another proving ground for the full combined Glamsterdam spec. Layer-2 teams, staking providers and other infrastructure operators all need working client builds in hand before they can safely run their own systems against the proposed fork.

Client Diversity Adds Another Layer of Complexity

Part of what makes this process tricky is Ethereum's multi-client architecture. The network runs on several independently built execution and consensus clients, and any upgrade has to function correctly across all the different possible client combinations. Even a bug isolated to just one client implementation can still disrupt an entire test network if the validators running that client carry enough combined weight.

Lido and Optimism Want a Stable Day Before Any Fork

According to the ACDC #186 agenda, both Lido and Optimism have asked for at least one full day of network stability before any fork goes live. The agenda also flagged outstanding client fixes and successful cross-client interoperability testing as prerequisites that need to be confirmed before Sepolia proceeds.

Hoodi Date on Hold Until Sepolia Firms Up

Developers have held off on setting a Glamsterdam date for Hoodi while Sepolia's own timeline is still up in the air. Hoodi is meant to function as the second public testnet phase, giving staking operators and protocol teams an environment that more closely mirrors actual mainnet conditions.

Teku developer Enrico del Fante backed the decision to hold off on a Hoodi date during the ACDC #186 call, pointing to the recent Devnet-9 issues as reason enough to give the process more breathing room following the Sepolia decision.

December Mainnet Launch Remains a Target

A December mainnet rollout is still on the table as a possibility, but it's not confirmed. Scheduling Sepolia for early October leaves enough runway on the calendar for a second public testnet phase and for client teams to prepare their releases, assuming testing doesn't hit any major delays along the way.

So far, developers haven't published a mainnet epoch, an activation timestamp, or a final client release schedule. There's also no formal deadline in place for deciding whether the Oct. 6 Sepolia date will actually hold.

What Comes Next: Devnet-11 Launch on Sept. 14

The next concrete milestone is the Devnet-11 launch, planned for Sept. 14. Client teams will be watching closely for finality behavior, cross-client compatibility, and whether the fixes rolled out after previous test rounds actually hold up, all of which will determine whether Sepolia can move forward on the current schedule.

What This Timeline Tells Us

Setting a Sepolia date before Glamsterdam had even run cleanly on a private devnet only makes sense if that date is treated as a planning anchor, not a commitment.

The roadmap already jumped from Devnet-10 to Devnet-11 within days of a validator-heavy network failing to finalize, a sign that developers are letting stability, not the calendar, set the real pace.

That's likely the right call for a network this size, especially with Lido and Optimism pushing for a full stable day before any fork. But it also means the December mainnet target is aspirational for now: if Devnet-11 hits its own snags, the Oct. 6 Sepolia date slips, and everything after it moves with it.