Fee Sponsorship

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.

Requirements

Before you can sponsor transactions, you need:

An API key — Required to authenticate your sponsorship requests and associate them with your app balance. Request an API key.
Enterprise Partnership — Reach out to the Relay team to become an Enterprise Partner. Learn more about the Enterprise Partner Program.
A Linked Funding Address — A wallet address must be linked to your API key. This is the address that will fund your app balance. Reach out to the relay team to link your wallet address.
Sufficient App Balance — Your app balance must have enough funds to cover the fees you wish to sponsor.

Once you’re set up, you can begin depositing funds and sponsoring transactions.

Funding Your App Balance

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

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.

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.

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

Examples

Calldata	Behavior
`0x012345abcdef0000000000000000000000000000000000000000012345abcdef`	Credits `msg.sender`
`0x012345abcdefd5c0d17ccb9071d27a4f7ed8255f59989b9aee0d012345abcdef`	Credits `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.

Solver Address

Deposits should be sent to the Relay solver on Base:

Network	Address
Base	`0xf70da97812cb96acdf810712aa562db8dfa3dbef`

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
})

Checking Your App Balance

Verify your available balance using the Get App Fee Balances API:

curl --location 'https://api.relay.link/app-fees/{your-wallet-address}/balances'

{
  "currency": {
    "chainId": 8453,
    "address": "0x0000000000000000000000000000000000000000",
    "symbol": "USDC",
    "name": "USDC",
    "decimals": 6
  },
  "amount": "75021651210714015",
  "amountFormatted": "0.075021651210714015",
  "amountUsd": "116.235557",
  "minimumAmount": "75021651210714015"
}

Sponsoring Transactions

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

Required Header

Header	Type	Description
`x-api-key`	`string`	Your API key, required to authenticate sponsorship requests and associate them with your app balance.

Sponsorship Parameters

Add these parameters to your Get Quote API request:

Parameter	Type	Description
`subsidizeFees`	`boolean`	Set to `true` to have the sponsor pay for all fees associated with the request, including gas topup amounts.
`maxSubsidizationAmount`	`string`	(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 sponsor pays up to the cap and the user pays the remainder.
`subsidizeRent`	`boolean`	(Optional) Set to `true` to sponsor Solana ATA rent fees. Requires `subsidizeFees` to also be enabled. See Solana Fee Sponsorship.
`depositFeePayer`	`string`	(Optional) A Solana address to pay deposit transaction fees for Solana origin transactions. See Solana Fee Sponsorship.
`sponsoredFeeComponents`	`string[]`	(Optional) You can also choose to sponsor specific Relay fees for your users using the `sponsoredFeeComponents` parameter. This allows integrators to selectively cover different fee types (such as gas fees or relayer fees) while still collecting app fees.

Deposit addresses do not support sponsoredFeeComponents. maxSubsidizationAmount is supported by deposit addresses but if the amount exceeds the maximum the user pays all fees and nothing is subsidized.

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

How `maxSubsidizationAmount` Works

The maxSubsidizationAmount parameter acts as a spending cap for fee sponsorship:

If the total fees are at or below this amount, the sponsor covers the full cost
If the total fees exceed this amount, the sponsor pays up to the cap and the user pays the remainder

This protects you from unexpectedly high costs during network congestion or volatile market conditions, while still providing partial sponsorship benefit to your users.

With partial fee sponsorship, users always receive some benefit from your sponsorship even when fees spike. For example, if you set a

5.00 cap and fees total

7.00, you pay

5.00 and the user pays

2.00.

Partial fee sponsorship is not supported for deposit addresses, if the fees exceed the maxSubsidizationAmount the user will cover the fees completely and nothing will be sponsored.

Solana Fee Sponsorship

Solana transactions involve unique cost structures that differ from EVM chains. In addition to standard transaction fees, Solana requires rent payments for creating new accounts, such as Associated Token Accounts (ATAs). Relay provides specific parameters to handle these costs.

Understanding Solana Rent

When a user receives a token on Solana for the first time, an Associated Token Account must be created to hold that token. This account creation requires a rent payment (approximately 0.002 SOL). Without sponsorship, this cost falls on the user.

Sponsoring Destination Rent with `subsidizeRent`

The subsidizeRent parameter allows you to sponsor the rent fees for ATA creation on Solana destination transactions.

Parameter	Type	Description
`subsidizeRent`	`boolean`	Set to `true` to have the sponsor pay for Solana rent fees associated with ATA creation on the destination chain.

The subsidizeRent parameter must be used in conjunction with subsidizeFees. There is currently no system for separating destination gas and rent fees. If you enable subsidizeRent without subsidizeFees, the rent will not be sponsored.

Important considerations:

When subsidizeFees is enabled but subsidizeRent is not, requests involving ATA creation will automatically fall back to user-paid fees. This is because rent sponsorship without explicit opt-in presents an exploitation vector.
The exploitation risk stems from the fact that users can close their ATA after the transaction completes and reclaim the rent. This effectively converts sponsored rent into a direct transfer of funds to the user.
Only enable subsidizeRent when you have a trusted relationship with your users or have implemented safeguards against abuse.

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": "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913",
    "destinationChainId": 792703809,
    "destinationCurrency": "EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v",
    "tradeType": "EXACT_INPUT",
    "recipient": "9aUn5swQzUTRanaaTwmszxiv89cvFwUCjEBv1vZCoT1u",
    "amount": "10000000",
    "subsidizeFees": true,
    "subsidizeRent": true,
    "maxSubsidizationAmount": "5000000"
  }'

In this example, a user bridges USDC from Base to Solana. Both subsidizeFees and subsidizeRent are enabled, ensuring the sponsor covers all destination fees including any ATA rent required for the recipient to receive USDC on Solana.

Sponsoring Origin Deposits with `depositFeePayer`

For transactions originating from Solana, you can designate an alternative account to pay the transaction fees and rent for the deposit transaction using depositFeePayer.

Parameter	Type	Description
`depositFeePayer`	`string`	A Solana address that will pay the transaction fees and rent for deposit transactions on Solana. This account must have sufficient SOL to cover these costs. Must be different from the `user` address.

The depositFeePayer parameter operates independently from the subsidizeFees and subsidizeRent parameters. It applies specifically to Solana origin transactions and does not draw from your app balance.

Important restrictions and security considerations for depositFeePayer:

The depositFeePayer and user addresses must be different. You cannot use the same wallet address for both parameters.
Signing order is critical. Relay returns a transaction that requires the integrator’s sponsoring wallet to sign first. The integrator must sign the transaction before passing it to the user. If the user receives an unsigned transaction, they could potentially modify it to drain the sponsoring wallet.
Avoid same-chain swaps with untrusted users. The depositFeePayer parameter is exploitable when used for same-chain swaps on Solana. Only use it for same-chain operations when the wallet is under your control.

When to use depositFeePayer:

When you want to provide a gasless experience for users depositing from Solana
When your application maintains a dedicated fee-paying wallet for Solana transactions
When you want to abstract away the complexity of SOL requirements from your users

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": "9aUn5swQzUTRanaaTwmszxiv89cvFwUCjEBv1vZCoT1u",
    "originChainId": 792703809,
    "originCurrency": "EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v",
    "destinationChainId": 8453,
    "destinationCurrency": "0x833589fcd6edb6e08f4c7c32d4f71b54bda02913",
    "tradeType": "EXACT_INPUT",
    "recipient": "0xF0AE622e463fa757Cf72243569E18Be7Df1996cd",
    "amount": "10000000",
    "depositFeePayer": "YourFeePayerSolanaAddressHere"
  }'

In this example, a user bridges USDC from Solana to Base. The depositFeePayer address will cover the Solana transaction fees for the deposit, allowing the user to transact without holding SOL.

Solana Sponsorship Summary

Parameter	Direction	Funding Source	Use Case
`subsidizeFees` + `subsidizeRent`	To Solana (destination)	App Balance	Sponsor destination gas and ATA rent when users receive tokens on Solana
`depositFeePayer`	From Solana (origin)	Specified Solana wallet	Sponsor origin transaction fees when users deposit from Solana

What Fee Sponsorship Does Not Cover

Fee sponsorship is designed to cover relay and execution costs, but certain fees remain outside its scope:

Origin chain gas fees — Users must pay the gas required to submit their transaction on the origin chain (unless using depositFeePayer for Solana origins).
App fees — Fees configured via the appFees parameter are not deducted from your sponsor balance. These are separate charges that accrue to the specified recipient addresses.
First-time approval transactions when using usePermit — The initial approval transaction required for permit-based flows is not covered by fee sponsorship. However, all subsequent requests after the approval will be sponsored.

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.

Best Practices

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

App Fees — Learn how to collect fees from your users
Get App Fee Balances — API reference for checking balances
Get Quote — API reference for quote requests
Relay Fees — Understanding Relay’s fee structure
Solana Support — Guide to depositing and withdrawing on Solana

​What is Fee Sponsorship?

​Requirements

​Funding Your App Balance

​Option 1: Use the Relay App UI

​Option 2: Direct On-Chain Deposit

​How It Works

​Examples

​Solver Address

​TypeScript Example

​Checking Your App Balance

​Sponsoring Transactions

​Required Header

​Sponsorship Parameters

​Example Request

​How maxSubsidizationAmount Works

​Solana Fee Sponsorship

​Understanding Solana Rent

​Sponsoring Destination Rent with subsidizeRent

​Example Request

​Sponsoring Origin Deposits with depositFeePayer

​Example Request

​Solana Sponsorship Summary

​What Fee Sponsorship Does Not Cover

​Fees Object

​Best Practices

​Related Resources

What is Fee Sponsorship?

Requirements

Funding Your App Balance

Option 1: Use the Relay App UI

Option 2: Direct On-Chain Deposit

How It Works

Examples

Solver Address

TypeScript Example

Checking Your App Balance

Sponsoring Transactions

Required Header

Sponsorship Parameters

Example Request

How `maxSubsidizationAmount` Works

Solana Fee Sponsorship

Understanding Solana Rent

Sponsoring Destination Rent with `subsidizeRent`

Example Request

Sponsoring Origin Deposits with `depositFeePayer`

Example Request

Solana Sponsorship Summary

What Fee Sponsorship Does Not Cover

Fees Object

Best Practices

Related Resources