Fees & Gas

How cross-chain messaging costs work, and what you need to configure.

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What You Pay to Send a Message

Every cross-chain message has two costs, both paid on the source chain at send time:

Cost	What it is	Paid in	Paid by
Source chain gas	Normal transaction gas for calling messageSend()	Native token (ETH, MATIC, etc.)	User or calling contract
Delivery fee	Covers validation and delivery to the destination chain	Native token, included as msg.value	User or calling contract

That's it for most chains. The msg.value you include with messageSend() covers everything on the destination side — VIA Labs handles execution there.

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Protocol Fee (ERC-20)

The VIA Gateway has a fee collector (FeeCollectorV1) that can charge an ERC-20 token fee per message (e.g., USDC). This is separate from the native gas costs above.

Currently this fee is set to zero on all chains. The mechanism exists but is not active.

If fees are ever enabled:

• Your contract auto-approves the fee token when you call setMessageGateway() — no manual approval needed
• Use setMaxFee() to cap the maximum fee per message — if the fee exceeds your cap, messageSend() reverts
• Setting setMaxFee(0) means no cap (accepts any fee amount)

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Gas Refunds (Destination Side)

When a relayer delivers your message on the destination chain, they spend gas. The gas refund system reimburses them from your contract's WETH balance.

For all chains except Ethereum mainnet, VIA Labs handles destination gas automatically — you don't need to do anything.

Ethereum Mainnet Exception

Ethereum gas is 10-100x more expensive than L2s and spikes unpredictably. If your contract receives messages on Ethereum mainnet, you need to:

1. Fund your contract with WETH — the relayer is reimbursed from this balance
2. Set a maxGas cap — limits how much a relayer can claim per message

// After deployment on Ethereum:
myContract.setMaxGas(0.05 ether);  // Cap at 0.05 WETH per message
// Then deposit WETH into your contract

How Gas Refunds Work

The gas refund system is trust-based. The relayer self-reports how much gas they spent — there is no on-chain gas metering. The flow:

1. Relayer calls process() on the destination gateway with a gasRefundAmount
2. GasRefundV1 checks the amount against your maxGas cap
3. If within cap, WETH is transferred from your contract to the relayer
4. If maxGas is exceeded, the entire delivery reverts with GasMaxCap

maxGas is your only protection. A typical message delivery costs 0.001–0.01 ETH in gas. Set maxGas to 2–5x your expected cost as a safety margin.

maxGas value	Behavior
0 (default)	No cap — dangerous. Relayer can claim up to your entire WETH balance.
Non-zero	Reverts if claimed amount exceeds cap.

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What Developers Need to Configure

Function	When to use	Default
setMaxFee(amount)	Cap the ERC-20 protocol fee per message	0 (no cap) — safe while fees are zero
setMaxGas(amount)	Cap the gas refund per message	0 (no cap) — set this if receiving on Ethereum

Both are called on your contract (inherited from ViaIntegrationV1). See the ViaIntegrationV1 reference for details.

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Summary

Scenario	What you pay	What you configure
Send from any chain	Source gas + msg.value for delivery	Nothing — just include msg.value
Receive on L2/non-Ethereum	Nothing	Nothing — VIA Labs covers destination gas
Receive on Ethereum mainnet	WETH from your contract balance	Fund WETH + setMaxGas()

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Testnet

Testnet fees are negligible. See Testnet Tokens for faucet links.

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Acceptable Use

VIA Labs reserves the right to throttle or suspend processing for traffic that is inorganic, malicious, or places excessive burden on relay infrastructure. This applies to both testnet and mainnet.