How to Design a Cross-Chain Gas Payment System

A cross-chain gas payment system solves a critical user experience problem in a multi-chain ecosystem: the need to hold native tokens (like ETH for Ethereum, MATIC for Polygon) on every network to pay for transactions. The core design goal is to enable a user on Chain A to initiate and pay for a transaction on Chain B using an asset they hold on Chain A. This requires a secure messaging protocol to relay the payment intent and a liquidity mechanism on the destination chain to cover the actual gas cost. Key architectural decisions involve choosing between a generalized messaging layer (like LayerZero, Axelar, Wormhole) or a custom bridge, and designing the fee abstraction logic on the target chain.

The system's security is paramount, as it involves moving value and executing logic across chains. You must design around the trust assumptions of your chosen messaging layer. For example, systems built on optimistic bridges have a delay for fraud proofs but lower cost, while those using light client or zk-based bridges offer faster finality with different cryptographic guarantees. The contract on the destination chain must validate incoming cross-chain messages, verify the user has provided sufficient prepayment on the source chain, and then execute the sponsored transaction. A common pattern is to use a gas tank or relayer network on the destination chain that is replenished by users' cross-chain payments.

Implementation involves three main smart contracts. First, a Source Chain Vault where users deposit their payment asset, locking it and emitting a message. Second, a Destination Chain Paymaster, a smart contract that adheres to standards like EIP-4337 for account abstraction or acts as a meta-transaction relayer. This contract receives the verified cross-chain message, validates the payment, and then executes the user's intended transaction, paying the native gas fees itself. Third, a Liquidity Management contract to handle the economics, ensuring the paymaster always has enough native tokens to sponsor transactions, often through automated rebalancing via the cross-chain bridge.

Consider the economic model and attack vectors. You must prevent gas price manipulation attacks where a user submits a transaction when gas prices spike, draining the paymaster. Implement gas price ceilings and transaction limits. Furthermore, design for liquidity efficiency; the paymaster shouldn't hold excessive idle capital. Solutions include using gas futures or dynamic fee pricing based on real-time cross-chain message costs. For developers, integrating with existing SDKs from cross-chain messaging providers significantly reduces complexity. For instance, Axelar's GasReceiver contract and LayerZero's Endpoint interface provide built-in patterns for cross-chain gas payment.

Real-world examples illustrate these patterns. The SocketDL infrastructure uses a liquidity pool model where users pay on a source chain, and a relayer executes on the destination. Biconomy's Hyphen bridge allows gasless transactions on a destination chain by prepaying on the source. When designing your system, start by defining the user flow: 1) User approves and deposits asset on Chain A, 2) A cross-chain message is sent with the target transaction calldata, 3) A relayer or off-chain service picks up the message, 4) The destination Paymaster validates and submits the transaction, paying gas. Thoroughly test all failure modes, including message reverts, liquidity shortfalls, and oracle failures on the destination chain.

A cross-chain gas payment system allows users to pay transaction fees on one blockchain using assets native to another. This requires a deep understanding of interoperability primitives like arbitrary message bridges (AMB), light clients, and relay networks. You should be familiar with how these components facilitate secure, trust-minimized communication between heterogeneous chains, such as Ethereum, Polygon, and Arbitrum. The core challenge is designing a system that maintains security and finality guarantees while handling gas token conversions.

Your development environment must be configured for multi-chain testing. Essential tools include Hardhat or Foundry for smart contract development, along with frameworks like Axelar, Wormhole, or LayerZero for cross-chain messaging. You will need access to testnet RPC endpoints for at least two chains (e.g., Sepolia and Polygon Mumbai) and faucets to obtain test tokens. Setting up a local relay or using a provider's SDK is necessary to simulate cross-chain message passing and fee payment flows in a controlled environment.

Smart contract security is paramount. Developers must be proficient in writing upgradeable contracts using patterns like Transparent Proxy or UUPS, as system logic will evolve. A strong grasp of gas optimization and reentrancy guards is required to minimize costs and vulnerabilities. You should also understand token standards like ERC-20 and ERC-677 for asset handling, and be prepared to audit the integration points with third-party bridges, which are common attack vectors.

The system's architecture depends on the chosen gas payment model. Will you use a meta-transaction relayer, a gas station network (GSN), or a lock-mint/burn-mint mechanism for the gas token? Each model has distinct prerequisites: meta-transactions require signature verification logic, a GSN needs a stake-managed relay hub, and a mint/burn system demands a secure, canonical token bridge. Your choice dictates the complexity of the smart contracts and the off-chain infrastructure you must build.

Finally, consider the operational requirements. You will need a method to monitor gas prices on destination chains to calculate accurate conversion rates, which may involve oracles like Chainlink. Planning for fee abstraction—allowing payment in stablecoins or the chain's native token—adds user flexibility but increases design complexity. Ensure your team is prepared to handle failed transactions and message reverts with clear recovery mechanisms to protect user funds.

The primary challenge in cross-chain gas payment is the atomic execution of two actions: receiving payment on the source chain and sponsoring a transaction on the destination chain. A naive approach would leave users vulnerable to paying for a transaction that never executes. The canonical solution is to use a relayer network or a decentralized protocol that acts as an intermediary. This entity pays the destination chain's gas fees upfront and is later reimbursed in the user's preferred token from the source chain. Systems like Gas Station Network (GSN) pioneered this concept for Ethereum, while newer protocols like Socket and Biconomy have extended it to a multi-chain environment.

Architecturally, the system requires several key smart contracts. On the destination chain, a Paymaster contract holds native gas tokens and logic to validate and sponsor user transactions. It must verify a cryptographic proof that the user has locked sufficient payment on the source chain. On the source chain, a Vault or Payment Router contract securely holds the user's payment tokens. A messaging layer, such as a canonical bridge (like Arbitrum's bridge), a general message passing protocol (like LayerZero or Axelar), or an optimistic verification system (like Hyperlane), is critical for securely communicating payment proofs between these contracts. The security of the entire system hinges on this cross-chain message reliability.

From a user's perspective, the flow is abstracted. They sign a meta-transaction on the destination chain, which is bundled with a proof of source-chain payment. A relayer submits this bundle. The Paymaster contract validates the proof via the cross-chain messaging layer. If valid, it pays the gas for the transaction execution. Once the message is confirmed on the source chain, the relayer (or a liquidity provider) is reimbursed from the user's locked funds, often minus a small fee. This requires maintaining liquidity pools of native gas tokens on various destination chains, which introduces economic considerations for relayers and liquidity providers.

When designing the Paymaster's validation logic, you must decide on a payment model. Common models include: a fixed-rate model where users pay a quoted fee in their token, a gas-token futures model where the relayer assumes price volatility risk, or a subscription model for frequent users. The validation must also include anti-abuse measures like rate-limiting, whitelisting of allowed to addresses, and replay protection across chains. Implementing EIP-4337's IPaymaster interface is becoming a standard for Ethereum and EVM chains, ensuring compatibility with a growing ecosystem of bundlers and wallets.

Testing and security are paramount. You must rigorously test the cross-chain message verification under adversarial conditions, including delayed messages, malicious relayer behavior, and destination chain reorgs. Use audit frameworks like Chainlink CCIP LocalSim or Hyperlane's Validator for local testing. Furthermore, the economic design must be stress-tested; insufficient liquidity on a destination chain will cause transaction failures, while poorly calibrated fees could make the system unsustainable or vulnerable to MEV extraction. A well-designed system significantly improves user experience by removing the need to manage multiple native tokens for gas.

The Gas Payment Vault is the foundational smart contract deployed on the destination blockchain (e.g., Arbitrum, Polygon). Its primary function is to custody the native gas token (like ETH on Arbitrum or MATIC on Polygon) that will be used to pay for user transactions. This contract must be upgradeable and secure, as it will hold significant value and be a central point of failure. Common patterns include using OpenZeppelin's TransparentUpgradeableProxy with an Ownable or AccessControl admin for managing upgrades and emergency functions.

The vault's logic handles the critical operation of releasing gas to pay for validated cross-chain requests. It exposes a function, typically payForTransaction, that can only be called by a pre-approved relayer or the verification module (built in Step 2). This function verifies an off-chain signature or a proof that the user has locked assets on the source chain before releasing a specified amount of native gas to cover the destination transaction's cost. Implementing a nonce or replay protection mechanism is essential to prevent the same proof from being used multiple times.

For security, the contract should include rate-limiting and emergency pause functions. A withdrawFees function allows the protocol admin to collect accumulated fees, which could be a small percentage of the gas paid or a flat fee. It's crucial to implement rigorous event logging for all gas payment transactions to ensure transparency and facilitate off-chain monitoring and accounting. The contract's initial deployment should be verified on block explorers like Etherscan for trust and auditability.

Here is a simplified skeleton of the vault's core function in Solidity:

solidityCopy
function payForTransaction(
    address user,
    uint256 amount,
    uint256 nonce,
    bytes calldata signature
) external onlyRelayer {
    require(!usedNonces[nonce], "Nonce already used");
    require(_verifySignature(user, amount, nonce, signature), "Invalid signature");
    
    usedNonces[nonce] = true;
    (bool success, ) = user.call{value: amount}("");
    require(success, "Gas transfer failed");
    
    emit GasPaid(user, amount, nonce);
}

This function checks a unique nonce, validates a cryptographic signature (proof of source-chain payment), and then sends the native amount to the user to sponsor their transaction.

After deployment, the vault must be funded with the destination chain's native token. This forms the protocol's gas liquidity pool. The required capital depends on anticipated user volume; protocols often start with a conservative amount and implement a rebalancing mechanism where relayers or keepers replenish the vault based on off-chain balance tracking. The next step is to build the verification system that authorizes these payments.

A cross-chain gas payment system requires a messaging protocol to communicate payment intent and execution results between chains. The core components are a Gas Oracle on the source chain to quote fees and a Gas Payment Executor on the destination chain to receive the payment and sponsor the user's transaction. Protocols like Axelar's General Message Passing (GMP), LayerZero, and Wormhole provide the underlying infrastructure for this secure message passing. Your design must integrate with one of these to relay the gas payment request.

The user flow begins on the source chain. Your dApp's frontend calls the Gas Oracle—a smart contract that calculates the estimated transaction cost on the destination chain in its native gas token. The user then approves and sends the equivalent amount in a supported bridged asset (e.g., USDC, WETH) to a designated vault contract. This vault, upon receiving funds, triggers the cross-chain message. The message payload must include critical data: the destination chain ID, the target contract address (Gas Payment Executor), the user's destination address, and the encoded calldata for the transaction to be sponsored.

On the destination chain, a relayer service monitors for these messages. Upon verification by the underlying messaging protocol's on-chain light client or oracle, the relayer submits the message to the Gas Payment Executor contract. This executor performs two key actions: it uses the bridged assets to swap for the native gas token via a DEX aggregator like 1inch or a liquidity pool, and then it calls the target contract as a gas sponsor, paying the fees on behalf of the user. Successful execution is typically relayed back to the source chain to update the user interface or trigger a refund for any unused gas.

Security is paramount. Implement circuit breakers and rate limits in your vault and executor contracts to prevent drain attacks. Use decentralized price oracles like Chainlink for accurate fee estimations and swap rates to avoid manipulation. Consider implementing a keeper network or a decentralized relayer model, as used by Gelato Network or Biconomy, to ensure reliable execution and avoid central points of failure. Always audit the integration with the underlying cross-chain messaging protocol for reentrancy and message verification vulnerabilities.

The core of a cross-chain gas payment service is a liquidity pool on each supported blockchain. When a user initiates a transaction on Chain A and pays for gas on Chain B, the service's relayer must have sufficient native tokens (like ETH, MATIC, or AVAX) on Chain A to submit the transaction. Each pool is funded with the chain's native token and is responsible for covering the gas costs of sponsored transactions. The initial pool size is determined by projected transaction volume and average gas costs, often starting with a multi-signature wallet controlled by the service operator for security.

Continuous usage creates liquidity imbalances. The pool on a high-activity chain will deplete, while the pool on the payment chain (where users pay) will accumulate excess funds. Without intervention, the service halts when any pool runs dry. An automated rebalancing mechanism is essential. This is typically implemented as a smart contract function that, when triggered by an off-chain keeper or oracle, initiates a cross-chain transfer of value from the surplus chain to the deficit chain using a bridge like Axelar, Chainlink CCIP, or a canonical bridge.

The rebalancing logic must account for bridge latency and costs. A simple threshold-based model works well: when a pool's balance falls below a minThreshold, a rebalancing transaction is queued to bring it up to a targetBalance. The amount to transfer is calculated as targetBalance - currentBalance + estimatedBridgeFee. These parameters should be adjustable via governance to adapt to changing network conditions. Using a gas price oracle (e.g., Chainlink's Gas Station) for the destination chain can make fee estimation more accurate, preventing underfunding.

For developers, implementing a basic rebalancer involves listening to pool balance events and calling a bridge protocol. Below is a simplified keeper script concept using the Axelar SDK:

javascriptCopy
// Pseudocode: Keeper checks and rebalances
const axelar = require('@axelar-network/axelarjs-sdk');
async function checkAndRebalance(poolAddress, chain) {
  const balance = await getNativeBalance(poolAddress, chain);
  if (balance < MIN_THRESHOLD) {
    const amtToSend = TARGET_BALANCE - balance + BRIDGE_FEE_ESTIMATE;
    await axelar.gateway.sendToken(
      PAYMENT_CHAIN, // Source chain with surplus
      chain,         // Destination chain with deficit
      poolAddress,   // Destination address
      amtToSend,
      'uusdc'        // Often use a stablecoin as bridge asset
    );
  }
}

Security and cost optimization are critical in rebalancing. Use multi-signature controls or a timelock for manual overrides. To minimize bridge fees, batch rebalancing operations instead of reacting to every threshold breach. Consider implementing a just-in-time liquidity model where user payments on the source chain are automatically converted and bridged ahead of their transaction, reducing the need for large, proactive rebalances. The choice of bridge impacts security, speed, and cost; canonical bridges are often more secure but less generic, while third-party bridges offer wider chain support with additional trust assumptions.

Finally, monitor pool health with dashboards tracking key metrics: balance vs. threshold on each chain, rebalancing transaction frequency and cost, and average time to replenish a depleted pool. Set up alerts for failed rebalancing attempts. Effective liquidity management transforms a cross-chain gas service from a prototype into a reliable, production-ready utility that can scale with user demand without manual treasury intervention.

You now have the architectural blueprint for a system that allows users to pay for gas on a destination chain using assets from a source chain. The key components are the Gas Payment Vault (a smart contract on the destination chain), a message relayer (like Axelar, Wormhole, or LayerZero), and a fee abstraction module that handles the conversion and payment logic. The next step is to implement the GasPaymentVault.sol contract, rigorously test its payForGas function, and integrate it with your chosen cross-chain messaging protocol.

For implementation, start by forking a testnet like Sepolia or Arbitrum Sepolia. Deploy your vault contract and a simple mock application (e.g., a basic NFT minting contract) that uses it. Use the official SDKs from your chosen interoperability protocol—such as the Axelar GMP SDK or Wormhole's Relayer Engine—to build the relayer service. This service will listen for GasPaid events on the source chain and execute the payload on the destination chain, funded by the vault.

Critical testing must cover edge cases: insufficient vault balance, failed message execution, and replay attacks. Use a framework like Foundry to write fuzz tests that simulate random gas prices and token amounts. Security audits are non-negotiable before mainnet deployment; consider engaging firms like OpenZeppelin or Trail of Bits. Furthermore, analyze the economic model: ensure the vault's token reserves are sufficiently liquid and that your fee-taking mechanism (if any) is sustainable against volatile gas prices.

Looking ahead, consider advanced features to integrate. Gas estimation oracles can provide real-time gas cost predictions to users before they initiate a cross-chain transaction. Support for ERC-20 fee tokens beyond a single stablecoin increases user flexibility. You could also explore implementing a gas sponsorship model, where dApps subsidize fees for their users, building the logic directly into your vault's allowance system.

The final step is integration and monitoring. Provide clear documentation for developers on how to modify their contracts to use your IGasPaymaster interface. Once live, use off-chain monitoring tools to track vault balances, relay success rates, and average gas costs. A well-designed cross-chain gas system removes a major UX barrier, enabling truly seamless multichain applications.