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What is Fee Sponsorship?

Fee Sponsorship allows you to cover the destination chain fees for your users’ Relay transactions. By sponsoring these fees, you can reduce costs for your users and create a more seamless experience. This is particularly valuable for onboarding new users or providing a premium experience for your application.
Fee sponsorship covers all destination chain fees (including gas topup amounts), but not the origin chain gas fee. Users will still need to pay the gas required to submit their transaction on the origin chain.

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Requirements

Before you can sponsor transactions, you need:
  1. An API Key — Required to authenticate your sponsorship requests and associate them with your app balance.
  2. A Linked Funding Address — A wallet address must be linked to your API key. This is the address that will fund your app balance.
  3. Sufficient App Balance — Your app balance must have enough funds to cover the fees you wish to sponsor.
To get started with fee sponsorship, reach out to the Relay team to set up an API key and link it to your funding address.
Once you’re set up, you can begin depositing funds and sponsoring transactions.

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Funding Your App Balance

There are two ways to deposit funds to your app balance:

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Option 1: Use the Relay App UI

The simplest way to deposit funds is through the Relay App Balance UI. This provides a user-friendly interface for managing your balance.

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Option 2: Direct On-Chain Deposit

You can programmatically deposit to your app balance by sending a transaction on Base to the Relay solver. This method involves appending specific calldata in place of the request ID to identify the deposit.

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How It Works

When you transfer funds to the solver using specific calldata, Relay treats it as a deposit to your app balance. This method uses a fixed 12-character prefix and suffix (012345abcdef) with the middle portion specifying which address to credit: Calldata Format: 
0x[prefix][address-to-credit][suffix]
  • Prefix: 012345abcdef
  • Address to Credit: The wallet address to deposit funds for (use 0000000000000000000000000000000000000000 for msg.sender)
  • Suffix: 012345abcdef

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Examples

CalldataBehavior
0x012345abcdef0000000000000000000000000000000000000000012345abcdefCredits msg.sender
0x012345abcdefd5c0d17ccb9071d27a4f7ed8255f59989b9aee0d012345abcdefCredits 0xd5c0d17ccb9071d27a4f7ed8255f59989b9aee0d
The ability to specify a different address is useful for building auto-topup systems or having more granular control over which accounts receive deposits.

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Solver Address

Deposits should be sent to the Relay solver on Base:
NetworkAddress
Base0xf70da97812cb96acdf810712aa562db8dfa3dbef

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TypeScript Example

import { erc20Abi, encodeFunctionData, createWalletClient } from 'viem'
import { base } from 'viem/chains'

const BASE_USDC_ADDRESS = '0x833589fcd6edb6e08f4c7c32d4f71b54bda02913' as const
const RELAY_BASE_SOLVER_ADDRESS = '0xf70da97812cb96acdf810712aa562db8dfa3dbef' as const
const REQUEST_ID_PREFIX = '012345abcdef'

const walletClient = createWalletClient({
  chain: base,
  transport: custom(window.ethereum!),
})

const [userAddress] = await walletClient.getAddresses()

const amountInWei = '100000000' // 100 USDC

// Encode the transfer function call
const transferData = encodeFunctionData({
  abi: erc20Abi,
  functionName: 'transfer',
  args: [RELAY_BASE_SOLVER_ADDRESS, amountInWei]
})

// Add custom calldata to credit the wallet address
const customCalldata =
  `${REQUEST_ID_PREFIX}${userAddress.slice(2)}${REQUEST_ID_PREFIX}` as `0x${string}`

const hash = await walletClient.sendTransaction({
  to: BASE_USDC_ADDRESS,
  data: `${transferData}${customCalldata}`,
  account: userAddress,
  chain: base
})

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Checking Your App Balance

Before sponsoring transactions, you’ll want to verify your available balance. Use the Get App Fee Balances API:
cURL
curl --location 'https://api.relay.link/app-fees/{your-wallet-address}/balances'
Example:
cURL
curl --location 'https://api.relay.link/app-fees/0x03508bb71268bba25ecacc8f620e01866650532c/balances'
Example Response: 
{
  "currency": {
    "chainId": 8453,
    "address": "0x0000000000000000000000000000000000000000",
    "symbol": "USDC",
    "name": "USDC",
    "decimals": 6,
    "metadata": {
      "logoURI": "https://assets.relay.link/icons/1/light.png",
      "verified": false,
      "isNative": false
    }
  },
  "amount": "75021651210714015",
  "amountFormatted": "0.075021651210714015",
  "amountUsd": "116.235557",
  "minimumAmount": "75021651210714015"
}

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Sponsoring Transactions

Once your app balance is funded, you can sponsor transactions by including specific parameters in your quote requests.

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Required Header

HeaderTypeDescription
x-api-keystringYour API key, required to authenticate sponsorship requests and associate them with your app balance.

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Sponsorship Parameters

Add these parameters to your Get Quote API request:
ParameterTypeDescription
subsidizeFeesbooleanSet to true to have the sponsor pay for all fees associated with the request, including gas topup amounts.
maxSubsidizationAmountstring(Optional) The maximum amount to subsidize in USDC with 6 decimal places (e.g., "1000000" = $1.00). If the total fees exceed this threshold, the entire request will not be subsidized.

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Example Request

cURL
curl -X POST \
  'https://api.relay.link/quote' \
  -H 'Content-Type: application/json' \
  -H 'x-api-key: your-api-key' \
  -d '{
    "user": "0xF0AE622e463fa757Cf72243569E18Be7Df1996cd",
    "originChainId": 8453,
    "originCurrency": "0x0000000000000000000000000000000000000000",
    "destinationChainId": 10,
    "destinationCurrency": "0x0000000000000000000000000000000000000000",
    "tradeType": "EXACT_INPUT",
    "recipient": "0xF0AE622e463fa757Cf72243569E18Be7Df1996cd",
    "amount": "100000000000000000",
    "subsidizeFees": true,
    "maxSubsidizationAmount": "5000000"
  }'
In this example:
  • x-api-key header authenticates your sponsorship request
  • subsidizeFees: true enables fee sponsorship
  • maxSubsidizationAmount: "5000000" caps sponsorship at $5.00 USDC

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How maxSubsidizationAmount Works

The maxSubsidizationAmount parameter acts as a safety threshold:
  • If the total fees are at or below this amount, the sponsor covers the full cost
  • If the total fees exceed this amount, no sponsorship occurs — the user pays all fees
This protects you from unexpectedly high costs during network congestion or volatile market conditions.
When maxSubsidizationAmount is exceeded, the entire request falls back to user-paid fees. The fees are not partially subsidized.

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Fees Object

When you sponsor transactions, the quote response includes a fees object with a subsidized field showing the fees covered by your sponsorship: 
{
  "fees": {
    "relayerService": "...",
    "relayerGas": "...",
    "relayer": "...",
    "app": "...",
    "subsidized": {
      "amount": "500000",
      "amountUsd": "0.50"
    }
  }
}
For more details on the fee structure, see Relay Fees.

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Best Practices

  1. Monitor Your Balance — Set up alerts or automated checks to ensure your app balance doesn’t run dry during high-traffic periods.
  2. Use maxSubsidizationAmount — Always set a reasonable cap to protect against unexpected fee spikes.
  3. Consider Auto-Topup — For production applications, consider implementing an auto-topup system using the direct on-chain deposit method.
  4. Track Sponsored Transactions — Use the Get Requests API to monitor your sponsored transaction history and costs.