Upgrade 16a: Protocol upgrade

Upgrade 16a is a maintenance release that supersedes Upgrade 16. If you are on upgrade 15 please skip upgrade 16 and go straight to this upgrade. It temporarily removes unused interop withdrawal-proving code introduced in Upgrade 16 and adds system-level feature toggles to improve flexibility and rollout safety. These changes reduce risk while maintaining momentum toward the Superchain roadmap.

The Upgrade 16a protocol upgrade on the Sepolia Superchain will be executed on Mon, Sept 22, 2025, and the Mainnet Superchain will be activated on Thur, Oct 2, 2025. The upgrade will be executed on the following chains: OP, Base, Soneium, Ink, and Unichain. Execution times may vary depending on the current state of each chain.

What's included in Upgrade 16a

Upgrade 16a introduces the following changes:

Removal of interop withdrawal-proving code: Code paths for interop withdrawals added in Upgrade 16 have been removed following partner feedback. ETHLockbox remains supported on chains already running U16. Chains moving directly from U15 → U16a will not adopt ETHLockbox until a future upgrade.

System-level feature toggles Added via the SystemConfig contract. ETHLockbox is the first feature placed behind a toggle. This mechanism enables chain-specific configuration and safer iterative upgrades.

Development-only feature flags Introduced to allow testing of interop functionality on non-production environments. These flags cannot be enabled on production chains.

OP Contracts Manager (OPCM) updates Updated to support both upgrade paths: U15 → U16a and U16 → U16a.

Impact summary

No chain downtime is expected, however if upgrading from U15, withdrawals will be invalidated (see below).
Withdrawal proofs created under U16 remain valid.
Chains still on U15 are affected in the same way described in the Upgrade 16 notice.
Chains upgrading directly from U15 → U16a will not adopt ETHLockbox until a future release.
System-level toggles improve operator control and make future upgrades easier to roll out.

For chain operators

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Withdrawal invalidation impacts only chains still on U15. Any pending L1 withdrawals on those chains that are not finalized before the upgrade executes will need to be reproven after the upgrade completes. Users must re-submit their proof transactions after the upgrade. Chains already on U16 are not affected. No funds are at risk.

If you're upgrading from Upgrade 15, follow the chain operator instructions for U16 (opens in a new tab).