GasRefundV1

Reimburses relayers for gas costs when they deliver cross-chain messages on the destination chain. The refund is paid from the project contract's WETH balance.

This is a trust-based system. The relayer self-reports the gas amount — there is no on-chain gas metering. maxGas is the project's only protection against over-refunding.

For the fee mechanics overview, see Fees & Gas.

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How Refunds Work

When the gateway calls refund(project, relayer, gasRefundAmount):

1. Resolve amount — check if there's a whitelist override for this project
2. Check cap — if maxGas[project] is set and the amount exceeds it, revert
3. Check balance — project must have enough WETH
4. Check allowance — project must have approved this contract (done automatically by setMessageGateway())
5. Transfer — send WETH to relayer (or unwrap to native ETH first)

Refund Resolution

whitelist[project]	Refund amount	Meaning
0 (default)	Relayer's claimed gasRefundAmount	Trust the relayer
9999	0	No refund (FREE constant)
Any other value	That exact value	Fixed refund, ignores relayer's claim

maxGas Cap

maxGas[project]	Behavior
0 (default)	No cap — dangerous. Relayer can claim up to the project's full WETH balance.
Non-zero	Reverts with GasMaxCap if refund exceeds cap.

Always set maxGas to a reasonable value. Typical delivery costs 0.001–0.01 ETH. Set to 2–5x expected cost.

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Delivery Modes

Mode	unwrap setting	How relayer gets paid	Gas cost
Direct	false	WETH transferred directly from project to relayer	Lower
Unwrapped	true	WETH transferred to GasRefundV1, unwrapped to ETH, sent to relayer	Higher

Most relayers prefer native ETH (unwrapped mode).

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Functions (Owner)

Function	Parameters	Description
setGasToken	address addr	Set the WETH address. Auto-approves for withdraw.
setUnwrap	bool unwrap_	true = unwrap WETH to ETH for relayers
setGateway	address addr	Update the authorized gateway address
setWhitelist	address addr, uint256 amount	Per-project fixed refund. 0 = use relayer's claim, 9999 = no refund

Functions (Project)

Function	Parameters	Description
setMaxGas	uint256 amount	Cap the refund per message. Called by the project contract (msg.sender). 0 = no cap.

Functions (Gateway)

Function	Parameters	Description
refund	address recipient, address relayer, uint256 gasRefundAmount	Reimburse relayer. Only callable by the gateway.

View Functions

Function	Returns	Description
getGasToken	address	WETH address. Used by ViaIntegrationV1.setMessageGateway() for auto-approval.

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Errors

Error	When
CallerMismatch	refund() called by non-gateway address
GasMaxCap	Refund exceeds maxGas[project]
InsufficientBalance	Project doesn't have enough WETH
InsufficientAllowance	Project hasn't approved enough WETH
NotRefunded	Native ETH transfer to relayer failed (unwrap mode)

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WETH Approval

Developers don't manually approve WETH. When ViaIntegrationV1.setMessageGateway() is called, it automatically:

1. Queries gateway.gasCollector() to get this contract's address
2. Queries gasCollector.getGasToken() to get the WETH address
3. Calls WETH.approve(gasCollector, type(uint256).max)

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Security Notes

• Gas refund failures are currently silently ignored by the gateway — if refund() reverts, the message is still delivered
• The relayer has no on-chain gas verification — maxGas is the sole protection
• Setting maxGas(0) means a relayer can drain the entire WETH balance in one message