Thanos is an L2 optimistic rollup (fork of Optimism) where the L2 creator designates an ERC20 token as the native currency on the L2 instead of ETH. This ERC20 token is referred as L2 native token.

Because the L2 native token is the native currency:

• All transaction fees are paid with it.
• Any L2 contract using the native currency will reference the L2 native token. Existing contracts assuming the native currency is ETH will need to be updated to reflect this change.
• Messaging to L2 accepts L2 native token.
• L2 native token cannot be changed once L2 native token is initialized.

Although there are some differences in contracts, the backend remains largely unchanged to ensure compatibility with Optimism. The low-level APIs remain unchanged to allow for future implementation of new Optimism features.

The L2 Native Token Bridge has been changed to be based on Optimism's contract. The code base we used is shown below.

• Optimism (OP Stack v1.7.7: Fjord Network Upgrade)
• Thanos

Difference between L2 Native Token Bridge & Optimism’s Custom Gas Token

Please note that Thanos’ L2 Native Token Bridge is different than the Custom Gas Token developed by Optimism. Thanos focuses on supporting ERC20 token as a native currency without supporting ETH as native currency.

Unique features in L2 Native Token Bridge compared to Custom Gas Token:

• Deposit & withdraw L2 native token is supported using L1StandardBridge, L2StandardBridge
• Have the features the StandardBridge and CrossDomainMessenger like replayability.
• Send messages and relay messages with non-zero native token in L1CrossDomainMessenger
• Allow deposit & withdraw ETH to StandardBridge or CrossDomainMessenger. Custom Gas Token not support them. It must revert when ETH is sent from the StandardBridge or CrossDomainMessenger.

Unsupported features in L2 Native Token Bridge compared to Custom Gas Token:

• ETH cannot be set as L2 native token
• However, WETH (wrapped ETH) is an ERC20 token that can be used as an L2 native token.