# OP Stack Upgrade 16a: Coming September/October Upgrade 16a brings new operator controls and streamlined upgrade paths to the Superchain. Here's what you need to know if you're a chain operator. ## The Rollout When: - **Sepolia testnet**: September 22, 2025 - **Mainnet**: October 2, 2025 Notable chains include: Base, Ink, OP, Soneium, and Unichain. If you're still on Upgrade 15, you can skip ahead and jump straight to 16a; you will get the latest improvements without the intermediate step. ## Key Improvements **System-level feature toggles**: Chain operators can now enable/disable specific features without full protocol upgrades. **Streamlined interop foundation**: The withdrawal-proving architecture has been refined, creating a cleaner foundation for future interoperability features. **Flexible upgrade paths**: The tooling now elegantly handles multiple upgrade scenarios, making future maintenance much smoother. ## What This Means for Your Chain **More control**: Feature toggles give you unprecedented flexibility in how and when you adopt new capabilities. **Safer rollouts**: Test features independently without committing to full protocol changes. **Future-proofing**: This architecture makes your chain more adaptable as the Superchain evolves. ## Important Technical Notes **No downtime expected**: The upgrade should complete without chain interruption. **Withdrawal considerations**: If you're upgrading from U15, any pending L1 withdrawals that aren't finalized before the upgrade will need to be re-proven afterward. Users must re-submit their proof transactions. **ETHLockbox status**: Chains already running U16 keep ETHLockbox support. Chains moving directly from U15 to U16a won't get ETHLockbox until a future upgrade. **If you're on U15**: Follow the same upgrade process as outlined in the U16 documentation. ### Additional Resources * [Upgrade 16a - docs.optimism.io](https://docs.optimism.io/notices/upgrade-16a) * [Optimism Protocol Roadmap 2025](https://github.com/orgs/ethereum-optimism/projects/117)