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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.

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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.

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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.

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For chain operators

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.

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Chain operators must complete the following tasks:

  • Update op-challenger to op-challenger/v1.5.1
  • If you’re building your own op-challenger software, then ensure that the --cannon-bin flag is set to cannon/v1.6.0 build. There’s no action needed if you’re using the official Optimism docker image.

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For permissionless fault proof enabled chains

Chains running permissionless fault proofs will need to deploy new dispute game contracts with new absolute prestates.

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Withdrawal flow changes

  1. There will be a one-time invalidation of all pending withdrawal proofs created on L1.
  2. Complete any pending withdrawals before the upgrade is executed
  3. Avoid creating new withdrawal proofs that would not become executable in time
  4. If a withdrawal was invalidated, submit a second withdrawal proof transaction on L1
This invalidation does not place any ETH or ERC-20 tokens at risk.