Optimism greenlights Super Root upgrade for OP Stack

Governance approved Upgrade 20 on Sept. 16 and Optimism applied the contract update to four OP Stack testnets on Sept. 17: OP Sepolia, Ink Sepolia, Soneium Minato and Unichain Sepolia. Mainnet activation is conditional on a successful seven-day testnet soak, with a target execution date of Sept. 24.

The upgrade replaces Output Root Dispute Games with Super Root Dispute Games. Super Root games use timestamps rather than L2 block numbers to identify the state being challenged. The timestamp reference makes it possible for the dispute system to represent multiple chains at the same timestamp.

Each chain’s dispute infrastructure remains separate for now. Permissioned chains will use game type 5 and permissionless fault-proof deployments will use game type 9. Optimism deployed the change to testnets first to avoid altering the security layer again when cross-chain functionality is later enabled.

Operators that run fault proofs and process withdrawals must update tooling before the L1 contract cutover. Components such as op-challenger and op-dispute-mon need Super Root support. op-proposer must switch to the new RPC and the appropriate game type when the upgraded contracts are executed. Existing Output Root configuration will remain available until in-flight games conclude, so running fault-proof infrastructure must handle both formats during the transition.

Developers building bridges and withdrawal tooling must adapt integrations that inspect dispute games directly. Applications can no longer assume a game’s root claim is a single-chain Output Root and must extract the chain-specific root from the Super Root structure. Projects using viem/op-stack for withdrawal proving require version 2.51.0 or later and must pass the initiating transaction’s L2 timestamp through the proving flow to create valid proofs under the new system.

Ordinary users do not need to take action. Previously submitted withdrawal proofs remain valid and the OptimismPortal will accept both dispute-game formats during the transition. Nodes that simply follow chain data do not require a new activation configuration, and no L2 hardfork is necessary to implement the dispute-format change.

The release also updates the contracts manager and SystemConfig, adding safeguards around upgrade sequencing and removing legacy configuration functions after issues identified during Upgrade 19.

If the four testnets complete a clean seven-day soak, Optimism plans to execute the upgrade on mainnet on or around Sept. 24. If issues appear during the soak, mainnet activation will be delayed until the team and operators are satisfied with test results.