Optimism's Upgrade 20: Super Root testnet Sept. 17, mainnet Sept. 24

Optimism governance finished voting on Upgrade 20 on Sept. 16, the day before a targeted Sept. 17 contract upgrade on four OP Stack testnets: OP Sepolia, Ink Sepolia, Soneium Minato and Unichain Sepolia. Mainnet execution is scheduled for Sept. 24, contingent on a healthy seven-day testnet soak.

The change swaps Output Root Dispute Games for Super Root Dispute Games. Output Root games anchor claims to a layer-2 block number; Super Root games anchor them to a timestamp. Optimism describes that as a prerequisite for interoperability, because Super Roots can eventually represent multiple chains at the same moment. Interoperability itself is still a later step: Upgrade 20 keeps each chain's dispute infrastructure separate, and each game still carries a single chain's Output Root.

What it means if you've used OP Mainnet

Nothing changes for you. Previously submitted withdrawal proofs remain valid, and OptimismPortal continues to accept both dispute-game formats.

What chain operators and developers have to do

Operators of fault-proof infrastructure, the systems that challenge incorrect state before withdrawals finalise back to Ethereum, have a short list of moving parts. op-challenger and op-dispute-mon need Super Root support before the L1 contract cutover, and op-proposer has to move to the new RPC and game type once the upgraded contracts execute. Permissioned chains use game type 5, permissionless fault-proof deployments use game type 9. Bridge and withdrawal integrations need to recognise the new game types and pull the right chain-specific root from the Super Root structure. Apps using viem/op-stack for withdrawal proving need version 2.51.0 or later and have to pass the L2 timestamp through the proving flow.

What to watch next

The four testnets are on the clock for seven days from Sept. 17, with mainnet following on Sept. 24 if the soak stays clean. The specific vote tally for the governance proposal (for-against counts, turnout and quorum on Optimism's Governor) was not published alongside the approval; the only public confirmation of passage so far is the Sept. 16 voting completion. Optimism's stated reason for the swap is forward-looking: by changing the dispute system now, the team is trying to avoid modifying that security layer again when cross-chain interoperability eventually turns on. The release also bundles smaller changes to the contracts manager and SystemConfig, including safeguards around upgrade sequencing and a cleanup of legacy configuration functions that came up during Upgrade 19.