diff --git a/packages/contracts-bedrock/src/L2/interfaces/IL2ToL2CrossDomainMessenger.sol b/packages/contracts-bedrock/src/L2/interfaces/IL2ToL2CrossDomainMessenger.sol
index 8b6b9d617cf1..cb8d1952de65 100644
--- a/packages/contracts-bedrock/src/L2/interfaces/IL2ToL2CrossDomainMessenger.sol
+++ b/packages/contracts-bedrock/src/L2/interfaces/IL2ToL2CrossDomainMessenger.sol
@@ -4,6 +4,19 @@ pragma solidity ^0.8.0;
 /// @title IL2ToL2CrossDomainMessenger
 /// @notice Interface for the L2ToL2CrossDomainMessenger contract.
 interface IL2ToL2CrossDomainMessenger {
+    /// @notice Mapping of message hashes to boolean receipt values. Note that a message will only
+    ///         be present in this mapping if it has successfully been relayed on this chain, and
+    ///         can therefore not be relayed again.
+    /// @param _msgHash message hash to check.
+    /// @return Returns true if the message corresponding to the `_msgHash` was successfully relayed.
+    function successfulMessages(bytes32 _msgHash) external view returns (bool);
+
+    /// @notice Retrieves the next message nonce. Message version will be added to the upper two
+    ///         bytes of the message nonce. Message version allows us to treat messages as having
+    ///         different structures.
+    /// @return Nonce of the next message to be sent, with added message version.
+    function messageNonce() external view returns (uint256);
+
     /// @notice Retrieves the sender of the current cross domain message.
     /// @return _sender Address of the sender of the current cross domain message.
     function crossDomainMessageSender() external view returns (address _sender);