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In this tutorial, you’ll learn how to bridge a custom ERC-20 token from Ethereum to an OP Stack chain using the Standard Bridge system. This tutorial is meant for developers who already have an existing ERC-20 token on Ethereum and want to create a bridged representation of that token on OP Mainnet. This tutorial explains how you can create a custom token that conforms to the IOptimismMintableERC20 interface so that it can be used with the Standard Bridge system. A custom token allows you to do things like trigger extra logic whenever a token is deposited. If you don’t need extra functionality like this, consider following the tutorial on Bridging Your Standard ERC-20 Token Using the Standard Bridge instead.
The Standard Bridge does not support fee on transfer tokens or rebasing tokens because they can cause bridge accounting errors.

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About OptimismMintableERC20s

The Standard Bridge system requires that L2 representations of L1 tokens implement the IOptimismMintableERC20 interface. This interface is a superset of the standard ERC-20 interface and includes functions that allow the bridge to properly verify deposits/withdrawals and mint/burn tokens as needed. Your L2 token contract must implement this interface in order to be bridged using the Standard Bridge system. This tutorial will show you how to create a custom token that implements this interface.

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Dependencies

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Get ETH on Sepolia and OP Sepolia

This tutorial explains how to create a bridged ERC-20 token on OP Sepolia. You will need to get some ETH on both of these testnets.
You can use this faucet to get ETH on Sepolia. You can use the Superchain Faucet to get ETH on OP Sepolia.

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Add OP Sepolia to your wallet

This tutorial uses Remix to deploy contracts. You will need to add the OP Sepolia network to your wallet in order to follow this tutorial. You can use this website to connect your wallet to OP Sepolia.

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Get an L1 ERC-20 token address

You will need an L1 ERC-20 token for this tutorial. If you already have an L1 ERC-20 token deployed on Sepolia, you can skip this step. Otherwise, you can use the testing token located at 0x5589BB8228C07c4e15558875fAf2B859f678d129 that includes a faucet() function that can be used to mint tokens.

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Create an L2 ERC-20 token

Once you have an L1 ERC-20 token, you can create a corresponding L2 ERC-20 token on OP Sepolia. This tutorial will use Remix so you can easily deploy a token without a framework like Hardhat or Foundry. You can follow the same general process within your favorite framework if you prefer. In this section, you’ll be creating an ERC-20 token that can be deposited but cannot be withdrawn. This is just one example of the endless ways in which you could customize your L2 token.
1

Open Remix

Navigate to Remix in your browser.
2

Create a new file

Click the 📄 (“Create new file”) button to create a new empty Solidity file. You can name this file whatever you’d like, for example MyCustomL2Token.sol.
3

Copy the example contract

Copy the following example contract into your new file:
// SPDX-License-Identifier: MIT
pragma solidity 0.8.20;

// Import the standard ERC20 implementation from OpenZeppelin
import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import { IERC165 } from "@openzeppelin/contracts/utils/introspection/IERC165.sol";

/**
* @title ILegacyMintableERC20
* @notice Legacy interface for the StandardL2ERC20 contract.
*/
interface ILegacyMintableERC20 {
  function mint(address _to, uint256 _amount) external;
  function burn(address _from, uint256 _amount) external;

  function l1Token() external view returns (address);
  function l2Bridge() external view returns (address);
}

/**
* @title IOptimismMintableERC20
* @notice Interface for the OptimismMintableERC20 contract.
*/
interface IOptimismMintableERC20 {
  function remoteToken() external view returns (address);
  function bridge() external view returns (address);
  function mint(address _to, uint256 _amount) external;
  function burn(address _from, uint256 _amount) external;
}

/**
* @title Simplified Semver for tutorial
* @notice Simple contract to track semantic versioning
*/
contract Semver {
  string public version;

  // Simple function to convert uint to string for version numbers
  function toString(uint256 value) internal pure returns (string memory) {
      // This function handles numbers from 0 to 999 which is sufficient for versioning
      if (value == 0) {
          return "0";
      }
      
      uint256 temp = value;
      uint256 digits;
      
      while (temp != 0) {
          digits++;
          temp /= 10;
      }
      
      bytes memory buffer = new bytes(digits);
      
      while (value != 0) {
          digits -= 1;
          buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
          value /= 10;
      }
      
      return string(buffer);
  }

  constructor(uint256 major, uint256 minor, uint256 patch) {
      version = string(abi.encodePacked(
          toString(major),
          ".",
          toString(minor),
          ".",
          toString(patch)
      ));
  }
}

/**
* @title MyCustomL2Token
* @notice A custom L2 token based on OptimismMintableERC20 that can be deposited 
*         from L1 to L2, but cannot be withdrawn from L2 to L1.
*/
contract MyCustomL2Token is IOptimismMintableERC20, ILegacyMintableERC20, ERC20, Semver {
  /// @notice Address of the corresponding token on the remote chain.
  address public immutable REMOTE_TOKEN;

  /// @notice Address of the StandardBridge on this network.
  address public immutable BRIDGE;

  /// @notice Emitted whenever tokens are minted for an account.
  /// @param account Address of the account tokens are being minted for.
  /// @param amount  Amount of tokens minted.
  event Mint(address indexed account, uint256 amount);

  /// @notice Emitted whenever tokens are burned from an account.
  /// @param account Address of the account tokens are being burned from.
  /// @param amount  Amount of tokens burned.
  event Burn(address indexed account, uint256 amount);

  /// @notice A modifier that only allows the bridge to call
  modifier onlyBridge() {
      require(msg.sender == BRIDGE, "MyCustomL2Token: only bridge can mint and burn");
      _;
  }

  /// @param _bridge      Address of the L2 standard bridge.
  /// @param _remoteToken Address of the corresponding L1 token.
  /// @param _name        ERC20 name.
  /// @param _symbol      ERC20 symbol.
  constructor(
      address _bridge,
      address _remoteToken,
      string memory _name,
      string memory _symbol
  )
      ERC20(_name, _symbol)
      Semver(1, 0, 0)
  {
      REMOTE_TOKEN = _remoteToken;
      BRIDGE = _bridge;
  }

  /// @notice Allows the StandardBridge on this network to mint tokens.
  /// @param _to     Address to mint tokens to.
  /// @param _amount Amount of tokens to mint.
  function mint(
      address _to,
      uint256 _amount
  )
      external
      virtual
      override(IOptimismMintableERC20, ILegacyMintableERC20)
      onlyBridge
  {
      _mint(_to, _amount);
      emit Mint(_to, _amount);
  }

  /// @notice Burns tokens from an account.
  /// @dev This function always reverts to prevent withdrawals to L1.
  /// @param _from   Address to burn tokens from.
  /// @param _amount Amount of tokens to burn.
  function burn(
      address _from,
      uint256 _amount
  )
      external
      virtual
      override(IOptimismMintableERC20, ILegacyMintableERC20)
      onlyBridge
  {
      // Instead of calling _burn(_from, _amount), we revert
      // This makes it impossible to withdraw tokens back to L1
      revert("MyCustomL2Token: withdrawals are not allowed");
      
      // Note: The following line would normally execute but is unreachable
      // _burn(_from, _amount);
      // emit Burn(_from, _amount);
  }

  /// @notice ERC165 interface check function.
  /// @param _interfaceId Interface ID to check.
  /// @return Whether or not the interface is supported by this contract.
  function supportsInterface(bytes4 _interfaceId) external pure virtual returns (bool) {
      bytes4 iface1 = type(IERC165).interfaceId;
      // Interface corresponding to the legacy L2StandardERC20
      bytes4 iface2 = type(ILegacyMintableERC20).interfaceId;
      // Interface corresponding to the updated OptimismMintableERC20
      bytes4 iface3 = type(IOptimismMintableERC20).interfaceId;
      return _interfaceId == iface1 || _interfaceId == iface2 || _interfaceId == iface3;
  }

  /// @notice Legacy getter for the remote token. Use REMOTE_TOKEN going forward.
  function l1Token() public view override returns (address) {
      return REMOTE_TOKEN;
  }

  /// @notice Legacy getter for the bridge. Use BRIDGE going forward.
  function l2Bridge() public view override returns (address) {
      return BRIDGE;
  }

  /// @notice Getter for REMOTE_TOKEN.
  function remoteToken() public view override returns (address) {
      return REMOTE_TOKEN;
  }

  /// @notice Getter for BRIDGE.
  function bridge() public view override returns (address) {
      return BRIDGE;
  }
}
4

Review the example contract

Take a moment to review the example contract. It’s closely based on the official OptimismMintableERC20 contract with one key modification:The burn function has been modified to always revert, making it impossible to withdraw tokens back to L1.Since the bridge needs to burn tokens when users want to withdraw them to L1, this means that users will not be able to withdraw tokens from this contract. Here’s the key part of the contract that prevents withdrawals:
  /// @notice Burns tokens from an account.
  /// @dev This function always reverts to prevent withdrawals to L1.
  /// @param _from   Address to burn tokens from.
  /// @param _amount Amount of tokens to burn.
  function burn(
      address _from,
      uint256 _amount
  )
      external
      virtual
      override(IOptimismMintableERC20, ILegacyMintableERC20)
      onlyBridge
  {
      // Instead of calling _burn(_from, _amount), we revert
      // This makes it impossible to withdraw tokens back to L1
      revert("MyCustomL2Token: withdrawals are not allowed");
      
      // Note: The following line would normally execute but is unreachable
      // _burn(_from, _amount);
      // emit Burn(_from, _amount);
  }
5

Compile the contract

Save the file to automatically compile the contract. If you’ve disabled auto-compile, you’ll need to manually compile the contract by clicking the “Solidity Compiler” tab (this looks like the letter “S”) and press the blue “Compile” button.Make sure you’re using Solidity compiler version 0.8.15 (the same version used in the official Optimism contracts).
6

Deploy the contract

Open the deployment tab (this looks like an Ethereum logo with an arrow pointing left). Make sure that your environment is set to “Injected Provider”, your wallet is connected to OP Sepolia, and Remix has access to your wallet. Then, select the MyCustomL2Token contract from the deployment dropdown and deploy it with the following parameters:
_bridge:      "0x4200000000000000000000000000000000000010" // L2 Standard Bridge address
_remoteToken: "<L1 ERC-20 address>"                        // Your L1 token address
_name:        "My Custom L2 Token"                         // Your token name
_symbol:      "MCL2T"                                      // Your token symbol
Note: The L2 Standard Bridge address is a predefined address on all OP Stack chains, so it will be the same on OP Sepolia and OP Mainnet.

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Bridge some tokens

Now that you have an L2 ERC-20 token, you can bridge some tokens from L1 to L2. Check out the tutorial on Bridging ERC-20 tokens with viem to learn how to bridge your L1 ERC-20 to L2s using viem. Remember that the withdrawal step will not work for the token you just created! This is exactly what this tutorial was meant to demonstrate.

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Add to the Superchain Token List

The Superchain Token List is a common list of tokens deployed on chains within the Optimism Superchain. This list is used by services like the Superchain Bridges UI. If you want your OP Mainnet token to be included in this list, take a look at the review process and merge criteria.