Glamsterdam compatibility notice for Arbitrum Sepolia chains
Ethereum Sepolia activates the Glamsterdam hard fork on Tuesday, October 6, 2026, at 13:53:36 UTC. This notice is parent chain compatibility guidance. It does not require an ArbOS upgrade.
If you run an Arbitrum Sepolia node, or a node for an Arbitrum chain that settles on Ethereum Sepolia, upgrade to Nitro v3.11.4 or higher before October 6, 2026.
- Docker image: offchainlabs/nitro-node:v3.11.4-7d5ac27
- Release notes: https://github.com/OffchainLabs/nitro/releases/tag/v3.11.4
Who is this notice for?
This notice applies to you if you run either of the following:
- An Arbitrum Sepolia node. Upgrade Nitro before the activation.
- An Arbitrum chain that settles on Ethereum Sepolia. Upgrade Nitro on every node of your chain, pause the batch poster around the activation, and read the batch poster and blob sections below.
This notice does not apply to:
- Arbitrum One, Arbitrum Nova, or Arbitrum chains that settle on Ethereum mainnet. Ethereum mainnet has no scheduled Glamsterdam activation date.
- Arbitrum chains that settle on another Arbitrum chain (L3s). Their parent chain is not Ethereum Sepolia.
Why the upgrade is required
Glamsterdam adds new fields to Ethereum block headers. Nitro v3.11.4 decodes these fields (EIP-7928 and EIP-7843) and reproduces the canonical parent chain block hash. Earlier Nitro releases do not.
Offchain Labs will publish a newer release, Nitro v3.12.1, after this date. Upgrade to it when it is available.
Pause the batch poster around the activation
If you run an Arbitrum chain that settles on Ethereum Sepolia, disable the batch poster about 15 minutes before the activation, at 13:38 UTC, and enable it again a few minutes after the fork.
The batch poster fixes its gas estimate when it first builds a batch transaction. A batch estimated before the fork and included after it can run out of gas under Glamsterdam's gas rules and keep failing.
Pausing has no impact on users. The sequencer keeps producing blocks, and the batches post once the batch poster is back on. To pause, set --node.batch-poster.enable=false. After the activation, set it back to true.
Batch poster reimbursement
Glamsterdam raises the parent chain gas cost of each batch post. Your chain uses the perBatchGasCharge parameter to calculate how much it reimburses the batch poster. The current value reflects pre-Glamsterdam costs, so after activation the batch poster can receive less in reimbursements than it spends on each batch.
On Sepolia, the difference is a small amount of testnet ETH. Do not change perBatchGasCharge on Sepolia. Offchain Labs will publish the value to use before Ethereum mainnet activates Glamsterdam. To learn how the parameter works, refer to Configure and optimize gas.
Blob configuration
If your chain posts blobs to Ethereum Sepolia, turn off the automatic fallback to calldata. Set both batch poster flags:
--node.batch-poster.post-4844-blobs=true
--node.batch-poster.ignore-blob-price=true
Chains that post calldata need no change. To learn more about these flags, refer to Enabling blob transactions for Arbitrum batch poster.
Important dates
| Date | Network upgrade | Affected audience |
|---|---|---|
| Tuesday, October 6, 2026 at 13:53:36 UTC | Ethereum Sepolia Glamsterdam hard fork | Node operators running Arbitrum Sepolia; operators of Arbitrum chains settling on Ethereum Sepolia |
The activation point on Ethereum Sepolia is:
| Field | Value |
|---|---|
| UTC time | October 6, 2026, 13:53:36 |
| Eastern time | October 6, 2026, 09:53:36 EDT |
| Epoch | 353,024 |
| Slot | 11,296,768 |
| UNIX timestamp | 1791294816 |
This notice covers Ethereum Sepolia only. For the Sepolia schedule and client releases, refer to the Glamsterdam testnet announcement and EIP-7773.