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Superchain interop is in active development. Some features may be experimental.
As of May 2025, the cost of 100 interop messages is just a few cents. Unless OP Stack transaction costs increase significantly, interop costs should not be a primary consideration in your implementation decisions.To see the current cost of gas, go to a block explorer and look at a recent transaction.
There are several factors that determine the cost of an interop transaction:

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CrossL2Inbox

This is the low level protocol used by all interop protocols, including L2ToL2CrossDomainMessenger.

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Initiating message

The initiating message is any log entry. A log entry emitted by Solidity code contains 1-4 topics (t) and unlimited unstructured data bytes (n). The gas cost is calculated as 375(t+1)+8n.

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Executing message

The executing message cost has several components:
  1. The cost of posting the transaction.
  2. The cost of hashing the message.
  3. The cost of CrossL2Inbox.validateMessage.
  4. The cost of using the message.
The first and second components depend on the log entry. CrossL2Inbox.validateMessage only requires a 32 byte hash of the log entry, but actually using it typically requires the information that has been hashed. Additionally, you must provide the CrossL2Inbox with the information needed to locate the log entry. This information is encoded in a five-member structure that requires 160 bytes (32 bytes × 5 members). Lastly, you need to call a function which requires a 4-byte selector. Therefore, the total bytes required is: 164 + 32t + n Where:
  • 164 = base overhead (160 bytes for the structure + 4 bytes for the function selector)
  • t = number of topics in the log entry
  • n = number of data bytes in the log entry
Every transaction posted costs at least 21,000 gas. The hashing operation costs approximately 30+0.2×<number of bytes>, which is negligible by comparison. We can usually ignore the memory expansion cost, unless the validating contract uses a really large amount of memory. The cost of using the message is beyond the scope here, because it depends on your application. The main cost drivers are the 21,000 gas transaction cost plus the cost of posting a 164+32t+n byte transaction.

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Cross domain messenger

This higher level protocol adds some expenses, mostly because replay protection requires storage, and writing to storage is a relatively expensive operation.

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Initiating message

The initiating message is sent by L2ToL2CrossDomainMessenger.sendMessage. This function writes to storage twice. It writes to specify that the hash has a sent message. This would typically be written to previously empty storage, so the cost is 22,100 gas. Then it increments the nonce value. Overwriting previously used storage (which means storage where the present value is not zero) only costs 5,000 gas. Hence, the gas cost for creating an initiating message is approximately 27,100 gas, plus minor overhead for log emission and contract operations. Note that this estimate excludes the 21,000 gas base transaction cost, which applies to all transactions.

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Executing message

If autorelay is turned on in a blockchain, then you don’t care about the cost of the executing message. The chain operator will bear the cost. If autorelay is not turned on, the executing message is a call to L2ToL2CrossDomainMessenger.relayMessage. The only storage operation here is noting the hash has been used for a message already. This is previously unwritten storage, so we can expect to pay the full 22,100 in gas. Plus, of course, the 21,000 that any transaction costs. All the other gas costs are negligible.

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Conclusion

Unless the message is extremely long, the cost of an interop message, taking both sides together, is unlikely to exceed 100,000 gas. At the time of writing, each gas unit costs approximately $3×10^-9, so it would take about thirty messages to add up to a full cent.

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Next steps

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