Optimism Adapters - Across Protocol Contracts

Overview

Across Protocol provides two adapters for OP Stack chains:

Optimism_Adapter - Legacy adapter for Optimism mainnet with custom bridge support
OP_Adapter - Modern adapter for all OP Stack chains (Base, Mode, Zora, etc.)

Both adapters use the OP Stack’s CrossDomainMessenger system to send messages and tokens from L1 to L2.

Optimism_Adapter

Overview

The original adapter for Optimism mainnet with support for custom token bridges (DAI, SNX). Location: contracts/chain-adapters/Optimism_Adapter.sol

Constructor

constructor(
    WETH9Interface _l1Weth,
    address _crossDomainMessenger,
    IL1StandardBridge _l1StandardBridge,
    IERC20 _l1Usdc,
    ITokenMessenger _cctpTokenMessenger
)

Parameters:

_l1Weth - WETH contract on L1 (unwrapped before bridging)
_crossDomainMessenger - L1CrossDomainMessenger system contract
_l1StandardBridge - L1StandardBridge for token transfers
_l1Usdc - USDC token address for CCTP bridging
_cctpTokenMessenger - Circle CCTP TokenMessenger contract

Constants

uint32 public constant L2_GAS_LIMIT = 200_000;

// Custom bridge addresses
address public constant DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
address public constant DAI_OPTIMISM_BRIDGE = 0x10E6593CDda8c58a1d0f14C5164B376352a55f2F;
address public constant SNX = 0xC011a73ee8576Fb46F5E1c5751cA3B9Fe0af2a6F;
address public constant SNX_OPTIMISM_BRIDGE = 0x39Ea01a0298C315d149a490E34B59Dbf2EC7e48F;

Core Functions

relayMessage()

Sends cross-chain messages via the CrossDomainMessenger.

function relayMessage(address target, bytes calldata message) external payable override {
    sendCrossDomainMessage(target, L2_GAS_LIMIT, message);
    emit MessageRelayed(target, message);
}

// Inherited from CrossDomainEnabled
function sendCrossDomainMessage(
    address _crossDomainTarget,
    uint32 _gasLimit,
    bytes calldata _message
) internal {
    getCrossDomainMessenger().sendMessage(_crossDomainTarget, _message, _gasLimit);
}

Key Points:

Uses CrossDomainEnabled helper contract
Messages are sent to target contract on L2
Gas limit is fixed at 200,000
No ETH is sent to target (msg.value is for gas only)

relayTokens()

Bridges tokens to Optimism with multi-bridge support.

function relayTokens(
    address l1Token,
    address l2Token,
    uint256 amount,
    address to
) external payable override {
    // 1. WETH: Unwrap and send native ETH
    if (l1Token == address(L1_WETH)) {
        L1_WETH.withdraw(amount);
        L1_STANDARD_BRIDGE.depositETHTo{ value: amount }(to, L2_GAS_LIMIT, "");
    }
    // 2. USDC: Use Circle CCTP
    else if (_isCCTPEnabled() && l1Token == address(usdcToken)) {
        _transferUsdc(to, amount);
    }
    // 3. Custom bridges for DAI and SNX
    else {
        address bridgeToUse = address(L1_STANDARD_BRIDGE);

        if (l1Token == DAI) bridgeToUse = DAI_OPTIMISM_BRIDGE;
        if (l1Token == SNX) bridgeToUse = SNX_OPTIMISM_BRIDGE;

        IERC20(l1Token).safeIncreaseAllowance(bridgeToUse, amount);
        
        // SNX has custom bridge interface
        if (l1Token == SNX) {
            SynthetixBridgeToOptimism(bridgeToUse).depositTo(to, amount);
        } else {
            IL1StandardBridge(bridgeToUse).depositERC20To(
                l1Token,
                l2Token,
                to,
                amount,
                L2_GAS_LIMIT,
                ""
            );
        }
    }
    emit TokensRelayed(l1Token, l2Token, amount, to);
}

Custom Token Bridges

DAI Bridge

L1 Address: 0x10E6593CDda8c58a1d0f14C5164B376352a55f2F DAI uses MakerDAO’s custom bridge instead of the standard bridge.

if (l1Token == DAI) bridgeToUse = DAI_OPTIMISM_BRIDGE;

Synthetix (SNX) Bridge

L1 Address: 0x39Ea01a0298C315d149a490E34B59Dbf2EC7e48F SNX uses Synthetix’s custom bridge with a different interface:

interface SynthetixBridgeToOptimism is IL1StandardBridge {
    function depositTo(address to, uint256 amount) external;
}

Bridge Selection Logic

OP_Adapter

Overview

Modern adapter for all OP Stack chains including Base, Mode, Zora, Lisk, and others. Supports both bridged USDC (via OP USDC Bridge) and native USDC (via CCTP). Location: contracts/chain-adapters/OP_Adapter.sol

Constructor

constructor(
    WETH9Interface _l1Weth,
    IERC20 _l1Usdc,
    address _crossDomainMessenger,
    IL1StandardBridge _l1StandardBridge,
    IOpUSDCBridgeAdapter _l1USDCBridge,
    ITokenMessenger _cctpTokenMessenger,
    uint32 _recipientCircleDomainId
)

Parameters:

_l1Weth - WETH contract on L1
_l1Usdc - USDC token address on L1
_crossDomainMessenger - L1CrossDomainMessenger for the target OP Stack chain
_l1StandardBridge - L1StandardBridge for the target chain
_l1USDCBridge - OP USDC Bridge adapter (for bridged USDC) or address(0)
_cctpTokenMessenger - CCTP TokenMessenger (for native USDC) or address(0)
_recipientCircleDomainId - Circle domain ID for the destination chain

Bridge Configuration Validation:

if (address(_l1Usdc) != zero) {
    bool opUSDCBridgeDisabled = address(_l1USDCBridge) == zero;
    bool cctpUSDCBridgeDisabled = address(_cctpTokenMessenger) == zero;
    // Bridged and Native USDC are mutually exclusive
    if (opUSDCBridgeDisabled == cctpUSDCBridgeDisabled) {
        revert InvalidBridgeConfig();
    }
}

Either OP USDC Bridge OR CCTP must be configured, but not both.

Core Functions

relayMessage()

Identical to Optimism_Adapter:

function relayMessage(address target, bytes calldata message) external payable override {
    sendCrossDomainMessage(target, L2_GAS_LIMIT, message);
    emit MessageRelayed(target, message);
}

relayTokens()

Supports WETH, USDC (bridged or native), and standard ERC20s:

function relayTokens(
    address l1Token,
    address l2Token,
    uint256 amount,
    address to
) external payable override {
    // 1. WETH: Unwrap to ETH
    if (l1Token == address(L1_WETH)) {
        L1_WETH.withdraw(amount);
        L1_STANDARD_BRIDGE.depositETHTo{ value: amount }(to, L2_GAS_LIMIT, "");
    }
    // 2. USDC: Use CCTP or OP USDC Bridge
    else if (l1Token == address(usdcToken)) {
        if (_isCCTPEnabled()) {
            // Native USDC via CCTP
            _transferUsdc(to, amount);
        } else {
            // Bridged USDC via OP USDC Bridge
            IERC20(l1Token).safeIncreaseAllowance(address(L1_OP_USDC_BRIDGE), amount);
            L1_OP_USDC_BRIDGE.sendMessage(to, amount, L2_GAS_LIMIT);
        }
    }
    // 3. Standard ERC20 tokens
    else {
        IERC20(l1Token).safeIncreaseAllowance(address(L1_STANDARD_BRIDGE), amount);
        L1_STANDARD_BRIDGE.depositERC20To(l1Token, l2Token, to, amount, L2_GAS_LIMIT, "");
    }
    emit TokensRelayed(l1Token, l2Token, amount, to);
}

OP USDC Bridge

Some OP Stack chains use bridged USDC instead of native USDC. Interface:

interface IOpUSDCBridgeAdapter {
    function sendMessage(
        address to,
        uint256 amount,
        uint32 minGasLimit
    ) external;
}

Example chains using bridged USDC:

Base (transitioning to native)
Mode
Zora

CrossDomainEnabled

Both adapters inherit from CrossDomainEnabled, a modified version of Optimism’s helper contract with immutable state variables for delegatecall compatibility. Location: contracts/chain-adapters/CrossDomainEnabled.sol

contract CrossDomainEnabled {
    // Immutable for delegatecall compatibility
    address public immutable MESSENGER;

    constructor(address _messenger) {
        MESSENGER = _messenger;
    }

    function getCrossDomainMessenger() internal virtual returns (ICrossDomainMessenger) {
        return ICrossDomainMessenger(MESSENGER);
    }

    function sendCrossDomainMessage(
        address _crossDomainTarget,
        uint32 _gasLimit,
        bytes calldata _message
    ) internal {
        getCrossDomainMessenger().sendMessage(_crossDomainTarget, _message, _gasLimit);
    }
}

Why Immutable? Since adapters are called via delegatecall from HubPool, storage variables would read HubPool’s storage slots. Immutable variables are embedded in bytecode and work correctly in delegatecall contexts.

CrossDomainMessenger Interface

interface ICrossDomainMessenger {
    function sendMessage(
        address _target,
        bytes calldata _message,
        uint32 _minGasLimit
    ) external payable;
    
    function xDomainMessageSender() external view returns (address);
}

Key Methods:

sendMessage() - Sends message from L1 to L2
xDomainMessageSender() - Returns original L1 sender (used by SpokePool to verify admin)

Admin Verification on L2

OP Stack SpokePools verify admin messages using xDomainMessageSender():

// In Optimism_SpokePool.sol
function _requireAdminSender() internal view override {
    require(
        msg.sender == address(MESSENGER) &&
        MESSENGER.xDomainMessageSender() == hubPool,
        "Only HubPool via messenger"
    );
}

Verification Steps:

Message sender must be the CrossDomainMessenger contract
The cross-domain sender must be the HubPool address

Supported Chains

Optimism_Adapter

Optimism (Chain ID: 10)

OP_Adapter

Base (Chain ID: 8453)
Mode (Chain ID: 34443)
Lisk (Chain ID: 1135)
Zora (Chain ID: 7777777)
Blast (uses custom Blast_Adapter variant)
And other OP Stack chains

Gas Considerations

Fixed Gas Limit

Both adapters use a fixed L2 gas limit of 200,000:

uint32 public constant L2_GAS_LIMIT = 200_000;

This is sufficient for:

Token transfers via standard bridge
Admin function calls to SpokePool
CCTP transfers

No ETH Balance Check

Unlike Arbitrum adapter, OP adapters don’t require pre-funding ETH. The msg.value sent with the transaction covers L2 gas costs.

Examples

Relay Admin Message to Base SpokePool

// On HubPool using OP_Adapter for Base
bytes memory functionData = abi.encodeCall(
    SpokePool.setEnableRoute,
    (originToken, destinationChainId, enabled)
);

hubPool.relaySpokePoolAdminFunction(
    8453,  // Base chain ID
    functionData
);

Bridge WETH to Optimism

// WETH is automatically unwrapped to ETH and bridged
hubPool.relayTokens(
    WETH_L1,
    WETH_OPTIMISM,  // Actually receives ETH on L2
    10 ether,
    spokePoolAddress
);

Bridge USDC to Base (Bridged USDC)

// Uses OP USDC Bridge if CCTP is not enabled
hubPool.relayTokens(
    USDC_L1,
    USDC_BASE_BRIDGED,
    5000e6,  // 5000 USDC
    spokePoolAddress
);

Optimism_SpokePool.sol - SpokePool for Optimism mainnet
Base_Adapter.sol - Specialized adapter for Base (if different)
Mode_Adapter.sol - Specialized adapter for Mode (if different)
Blast_Adapter.sol - Custom adapter for Blast with yield features

​Overview

​Optimism_Adapter

​Overview

​Constructor

​Constants

​Core Functions

​relayMessage()

​relayTokens()

​Custom Token Bridges

​DAI Bridge

​Synthetix (SNX) Bridge

​Bridge Selection Logic

​OP_Adapter

​Overview

​Constructor

​Core Functions

​relayMessage()

​relayTokens()

​OP USDC Bridge

​CrossDomainEnabled

​CrossDomainMessenger Interface

​Admin Verification on L2

​Supported Chains

​Optimism_Adapter

​OP_Adapter

​Gas Considerations

​Fixed Gas Limit

​No ETH Balance Check

​Examples

​Relay Admin Message to Base SpokePool

​Bridge WETH to Optimism

​Bridge USDC to Base (Bridged USDC)

​Related Contracts

​Source Code

Overview

Optimism_Adapter

Overview

Constructor

Constants

Core Functions

relayMessage()

relayTokens()

Custom Token Bridges

DAI Bridge

Synthetix (SNX) Bridge

Bridge Selection Logic

OP_Adapter

Overview

Constructor

Core Functions

relayMessage()

relayTokens()

OP USDC Bridge

CrossDomainEnabled

CrossDomainMessenger Interface

Admin Verification on L2

Supported Chains

Optimism_Adapter

OP_Adapter

Gas Considerations

Fixed Gas Limit

No ETH Balance Check

Examples

Relay Admin Message to Base SpokePool

Bridge WETH to Optimism

Bridge USDC to Base (Bridged USDC)

Related Contracts

Source Code