Optimism Upgrade 20 changes fault proofs to super roots while retaining output-root withdrawal proofs

What happened

Optimism announced Upgrade 20 as shipping with a revised fault-proof system based on multi-chain-capable super roots. The upgrade changes dispute-game sequencing from L2 block numbers to timestamps and adds an invalid-state path for claims extending beyond available L1 data.

What changed

Fault-proof games now use super-root claims and timestamp-based sequence numbers. Chain operators must update proposer, challenger, and dispute-monitor configuration, and withdrawal tooling must select a game whose sequence number is at least the initiating L2 block's timestamp.

What did not change

OptimismPortal2.proveWithdrawalTransaction remains unchanged. Withdrawals continue to use an output-root proof and Merkle proof, with the portal obtaining the chain-specific output root from the super-root game. The upgrade does not itself enable cross-chain interoperability.

Confirmed

• Upgrade 20 replaces single-chain root claims in dispute games with super-root claims.
• The game sequence number is now a timestamp rather than an L2 block number.
• The canonical portal's withdrawal-proof function is unchanged.
• Node operators require no action, while chain operators must update fault-proof component configuration.
• Cross-chain messaging remains a future hard-fork feature.

Still open

• The exact activation state and deployment transaction for each OP Mainnet bridge contract were not included in the cached notice.
• Whether all relevant third-party withdrawal tooling has upgraded to timestamp-aware game selection.
• Operational performance after the upgraded games have accumulated production history.

What it means for an advisor

• Review the Optimism bridge memo's fault-proof and withdrawal-monitoring description.
• Verify the deployed implementation and activation transaction before recording the change as executed on-chain.
• Confirm that any operational withdrawal tooling uses timestamp-compatible libraries and monitor early production dispute games.

Previous interpretation

The canonical bridge review assumed single-chain output-root dispute games selected using L2 block-number sequencing.