> ## Documentation Index
> Fetch the complete documentation index at: https://docs.relay.link/llms.txt
> Use this file to discover all available pages before exploring further.

# Fee Sponsorship

> Sponsor your users' transaction fees to improve UX and reduce friction

## 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.

<Note>
  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.
</Note>

## 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. [Request an API key](/references/api/api-keys#how-to-get-an-api-key).
2. **Enterprise Partnership** — Reach out to the Relay team to become an Enterprise Partner. Learn more about the [Enterprise Partner Program](/resources/enterprise).
3. **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](https://forms.gle/XNeELYavjwoJwPEf7) to the relay team to link your wallet address.
4. **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](https://www.relay.link/app-balance). 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` |

<Tip>
  The ability to specify a different address is useful for building auto-topup systems or having more granular control over which accounts receive deposits.
</Tip>

#### Solver Address

Deposits should be sent to the Relay solver on Base:

| Network | Address                                      |
| ------- | -------------------------------------------- |
| Base    | `0xf70da97812cb96acdf810712aa562db8dfa3dbef` |

#### TypeScript Example

```typescript expandable theme={null} theme={null} theme={null} theme={null} theme={null}
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](/references/api/get-app-fee-balances) API:

<CodeGroup>
  ```bash Request theme={null} theme={null} theme={null} theme={null} theme={null}
  curl --location 'https://api.relay.link/app-fees/{your-wallet-address}/balances'
  ```

  ```json Response theme={null} theme={null} theme={null} theme={null} theme={null}
  {
    "currency": {
      "chainId": 8453,
      "address": "0x0000000000000000000000000000000000000000",
      "symbol": "USDC",
      "name": "USDC",
      "decimals": 6
    },
    "amount": "75021651210714015",
    "amountFormatted": "0.075021651210714015",
    "amountUsd": "116.235557",
    "minimumAmount": "75021651210714015"
  }
  ```
</CodeGroup>

***

## 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](/references/api/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](#solana-fee-sponsorship).                                                                                                   |
| `depositFeePayer`        | `string`   | *(Optional)* A Solana address to pay deposit transaction fees for Solana origin transactions. See [Solana Fee Sponsorship](#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. |

<Warning>
  [Deposit addresses](/features/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.
</Warning>

### Example Request

```bash cURL theme={null}
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.

<Info>
  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.
</Info>

<Warning>
  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.
</Warning>

***

## 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. |

<Warning>
  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.
</Warning>

**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

```bash cURL theme={null}
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.** |

<Note>
  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.
</Note>

<Warning>
  **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.
</Warning>

**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

```bash cURL theme={null}
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:

```json theme={null}
{
  "fees": {
    "relayerService": "...",
    "relayerGas": "...",
    "relayer": "...",
    "app": "...",
    "subsidized": {
      "amount": "500000",
      "amountUsd": "0.50"
    }
  }
}
```

For more details on the fee structure, see [Relay Fees](/references/api/api_core_concepts/fees).

***

## 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 onchain deposit method.

4. **Track Sponsored Transactions** — Use the [Get Requests](/references/api/get-requests) API to monitor your sponsored transaction history and costs.

***

## Related Resources

* [App Fees](/features/app-fees) — Learn how to collect fees from your users
* [Get App Fee Balances](/references/api/get-app-fee-balances) — API reference for checking balances
* [Get Quote](/references/api/get-quote) — API reference for quote requests
* [Relay Fees](/references/api/api_core_concepts/fees) — Understanding Relay's fee structure
* [Solana Support](/references/api/api_guides/solana) — Guide to depositing and withdrawing on Solana