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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.
1

Verify the new absolute prestate
2

As of upgrade 14, the 64 bit multi-threaded version of cannon is utilized.
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The absolute prestate is generated with the op-program/v1.6.1-rc.1. You can use this new absolute prestate 0x03eb07101fbdeaf3f04d9fb76526362c1eea2824e4c6e970bdb19675b72e4fc8 for the following chains:
4

  • Mainnet and Sepolia: OP, Base, Soneium, Inkand Unichain
    • 5

    You can verify this absolute prestate by running the following command in the root of the monorepo on the op-program/v1.6.1-rc.1 tag:
    6

    make reproducible-prestate
    7

    This will output the calculated prestates, which will look something like:
    8

    -------------------- Production Prestates --------------------


Cannon64 Absolute prestate hash: 
0x03eb07101fbdeaf3f04d9fb76526362c1eea2824e4c6e970bdb19675b72e4fc8

-------------------- Experimental Prestates --------------------

CannonInterop Absolute prestate hash: 
0x03fc3b4d091527d53f1ff369ea8ed65e5e17cc7fc98ebf75380238151cdc949c

Cannon64Next Absolute prestate hash: 
0x03eb07101fbdeaf3f04d9fb76526362c1eea2824e4c6e970bdb19675b72e4fc8
    9

  • The “Cannon64” hash is the 64-bit prestate.
    • 10

    Verify that your target prestate was calculated as expected and matches the corresponding entry in standard-prestates.toml.
    11

    Upload your new preimage file
    12

    During the previous step, you also generated the preimage of the absolute prestate, which is the op-program serialized into a binary file. You’ll find that new file at optimism/op-program/bin/prestate-mt64.bin.gz. Rename that file to have the absolute prestate hash as the filename so it looks like PRESTATEHASH.bin.gz.
    13

    Upload that file to where you’re storing your other absolute preimage files. This should be the location where you’re pointing your --cannon-prestates-url at. The op-challenger will grab this file and use it when it needs to challenge games.
    14

    Execute the upgrade
    15

    Once your op-challenger is ready with the new preimage, you can execute the upgrade transaction. This should be done by making a delegatecall to the upgrade() function of the OP Contract Manager (at the address listed in the registry.
    16

    Please simulate and validate the expected output prior to executing the transaction.

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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.
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