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

> 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.

- URL: https://riadefi.com/on-chain/2026-09-25-optimism-upgrade-20-super-root-fault-proofs/
- Type: Upgrade or migration
- Stage: Confirmed
- Evidence: confirmed
- Materiality: material
- Event date: 2026-09-25
- Version: 1, published 2026-09-27 (first published 2026-09-27)
- Follow-up: 2026-10-02
- Advisor-relevant: no
- Affects: Optimism canonical bridge (protocol)

## 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.

## Sources

1. [Upgrade 20: Preparing the Fault Proof System for Interop](https://optimism.io/blog/upgrade-20-preparing-the-fault-proof-system-for-interop) · Optimism Foundation · 2026-09-25


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Published by Ketju Research on RIADeFi (https://riadefi.com). Educational research for financial professionals; not investment, legal, tax, or compliance advice.