Optimism has approved Upgrade 20, moving its fault-proof system towards the architecture needed for interoperability across its Superchain.
Governance voting on the Ethereum layer-2 network concluded on 16 September, putting OP Sepolia, Ink Sepolia, Soneium Minato and Unichain Sepolia on course for a contract upgrade targeted for 17 September. Mainnet execution is scheduled for 24 September, provided the revised system completes a successful seven-day testnet run.
The upgrade replaces Output Root Dispute Games with Super Root Dispute Games. These systems are used by OP Stack chains to challenge incorrect state before withdrawals are finalised back to Ethereum.
Optimism says the change is a prerequisite for interoperability because Super Roots could eventually represent several chains at the same timestamp. However, Upgrade 20 will not make those chains interoperable immediately.
Each chain will continue to operate its own dispute infrastructure, with every new game still containing the Output Root of an individual chain. Optimism is making the change now to avoid having to alter the security layer again when cross-chain interoperability is introduced.
The release also includes updates to the contracts manager and `SystemConfig`. Those changes add safeguards for the order in which upgrades are carried out and remove legacy configuration functions following issues identified during Upgrade 19.
How Super Roots will affect withdrawals
The principal operational change concerns the way the system identifies the state being challenged.
Output Root games currently link a claim to an L2 block number. Super Root games will instead use a timestamp, providing the shared time reference required by a future multi-chain dispute system. Permissioned chains will use game type 5, while permissionless fault-proof deployments will use game type 9.
Operators must update their fault-proof infrastructure ahead of the L1 contract change. `op-challenger` and `op-dispute-mon` need to support Super Roots before the transition, while `op-proposer` must switch to the new RPC and game type when the upgraded contracts are executed.
Existing configuration will also need to remain available until older Output Root games have been completed.
Bridge and withdrawal applications face corresponding changes. Software that reads dispute games directly can no longer assume that every root claim represents a single-chain Output Root. Integrations must support the new game types and obtain the relevant chain-specific root from the Super Root structure.
Applications 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 process. Users do not need to take action: previously submitted withdrawal proofs remain valid, and OptimismPortal will continue to accept both dispute-game formats.
The transition changes core security infrastructure without requiring an L2 hardfork. OP nodes that only follow the chain need no new activation configuration, leaving most of the migration work to operators involved in fault proofs and withdrawals.
A clean seven-day testnet run would allow the targeted 24 September mainnet execution, moving OP Mainnet and other participating chains to the dispute format Optimism plans to use when Superchain interoperability reaches production.
