​
Non-Sponsored Calls

Non-Sponsored calls are also known as Sync fee Calls. It is the simplest way to pay, but it delegates all security (reentrancy/replay protection etc.) and payment logic to the target smart contract. You can use ERC-2771 to achieve out-of-the-box security and authentication. Relay costs are covered in either native or ERC-20 tokens and they are paid synchronously during the relay call.

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Implementation Steps

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1. Deploy a GelatoRelayContext compatible contract

Import GelatoRelayContext in your target contract to inherit callWithSyncFee functionalities. 
import {
    GelatoRelayContext
} from "@gelatonetwork/relay-context/contracts/GelatoRelayContext.sol";

contract TargetContract is GelatoRelayContext {

    function example() external onlyGelatoRelay {
        // _yourAuthenticationLogic()

        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelay modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
        _transferRelayFee();
    }

    function exampleFeeCapped(uint256 maxFee) external onlyGelatoRelay {
        // Remember to autheticate your call since you are not using ERC-2771
        // _yourAuthenticationLogic()

        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelay modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
        _transferRelayFeeCapped(maxFee);
    }
}
Import GelatoRelayContextERC2771 in your target contract to inherit ERC2771 functionalities with callWithSyncFee. Learn more about ERC2771 here. 
import {
    GelatoRelayContextERC2771
} from "@gelatonetwork/relay-context/contracts/GelatoRelayContextERC2771.sol";

contract TargetContractRelayContextERC2771 is GelatoRelayContextERC2771 {

    mapping (address=>bool) public caller;

    function increment() external onlyGelatoRelayERC2771 {

        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelayERC2771 modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
        _transferRelayFee();
        // _getMsgSender() will fetch the original user who signed the relay request.
        caller[_getMsgSender()] = true;
    }
    
    function incrementFeeCapped(uint256 maxFee) external onlyGelatoRelayERC2771 {

        // Payment to Gelato
        // NOTE: be very careful here!
        // if you do not use the onlyGelatoRelayERC2771 modifier,
        // anyone could encode themselves as the fee collector
        // in the low-level data and drain tokens from this contract.
  
        _transferRelayFeeCapped(maxFee);
        // _getMsgSender() will fetch the original user who signed the relay request.
        caller[_getMsgSender()] = true;
    }
}

​
2. Import GelatoRelaySDK into your front-end .js project

import { GelatoRelay, CallWithSyncFeeRequest } from "@gelatonetwork/relay-sdk";
const relay = new GelatoRelay();
or If you’re using the Viem library in your project, consider importing @gelatonetwork/relay-sdk-viem. 
import { GelatoRelay, CallWithSyncFeeRequest } from "@gelatonetwork/relay-sdk-viem";
const relay = new GelatoRelay();

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For ERC2771 Sync Fee Calls

import { GelatoRelay, CallWithSyncFeeERC2771Request } from "@gelatonetwork/relay-sdk-viem";
Once we have imported the GelatoRelay class, when using ERC2771 methods, we must initialize it with the appropriate trustedForwarder. The possible configurations are: 
contract: {
    relay1BalanceERC2771: "trustedForwarder for method sponsoredCallERC2771",
    relayERC2771: "trustedForwarder for method callWithSyncFeeERC2771",
    relay1BalanceConcurrentERC2771: "trustedForwarder for method concurrent sponsoredCallERC2771",
    relayConcurrentERC2771:"trustedForwarder for method concurrent callWithSyncFeeERC2771",
    relay1BalanceConcurrentERC2771zkSync: "trustedForwarder for method concurrent sponsoredCallERC2771 on zkSync",
    relay1BalanceERC2771zkSync: "trustedForwarder for method sponsoredCallERC2771 on zkSync",
    relayConcurrentERC2771zkSync: "trustedForwarder for method concurrent callWithSyncFeeERC2771 on zkSync",
    relayERC2771zkSync: "trustedForwarder for method callWithSyncFeeERC2771 on zkSync",
}
We will need to go to the Supported Networks and check the network and the contract addresses to identify the trustedForwarder associated with our method. In the example below, we are using the method callWithSyncFeeERC2771 on Sepolia, the trustedForwarder associated is 0xb539068872230f20456CF38EC52EF2f91AF4AE49. We will initialize GelatoRelay with the following config: 
const relay = new GelatoRelay({
    contract: {
        relayERC2771:"0xb539068872230f20456CF38EC52EF2f91AF4AE49"
    }
});

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3. Generate a payload for your target contract

// set up target address and function signature abi
const targetContractAddress = "<your target contract address>"; 
const abi = ["function example()","function exampleFeeCapped(uint256)"];

const [address] = await window.ethereum!.request({
      method: "eth_requestAccounts",
 });

// generate payload using front-end provider such as MetaMask
const client = createWalletClient({
      account: address,
      chain,
      transport: custom(window.ethereum!),
});

// address of the token to pay fees
const feeToken = "0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE";

const chainId = await client.getChainId();

//encode function data
const data = encodeFunctionData({
    abi: abi,
    functionName: "example",
});

// -----------------------------------------------------------------
// the following is an alternative example using Gelato Fee Oracle, 
// setting maxFee, and calling the exampleFeeCapped(maxFee) method

// retrieve the estimate fee from the Gelato 
const fee = await relay.getEstimatedFee(
  BigInt(chainId),
  feeToken,
  gasLimit,
  false,
)

const maxFee = fee * 2 // you can use 2x or 3x to set your maxFee

//encode function data
const data = encodeFunctionData({
    abi: abi,
    functionName: "exampleFeeCapped",
    args : [maxFee]
});

​
4. Send payload to Gelato

// populate the relay SDK request body
const request: CallWithSyncFeeRequest = {
  chainId: BigInt(chainId),
  target: targetContractAddress,
  data: data,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponse = await relay.callWithSyncFee(request);

// -----------------------------------------------------------------
// the following is an alternative example using Gelato Fee Oracle, 
// setting maxFee, and calling the exampleFeeCapped(maxFee) method

// populate the relay SDK request body
const requestMaxFee: CallWithSyncFeeRequest = {
  chainId: BigInt(chainId),
  target: targetContractAddress,
  data: dataMaxFee,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponse = await relay.callWithSyncFee(requestMaxFee);

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For ERC2771 Sync Fee Calls

// populate the relay SDK request body
const request: CallWithSyncFeeERC2771Request = {
  chainId: BigInt(chainId),
  target: targetContractAddress,
  data: data,
  user: address,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponse = await relay.callWithSyncFeeERC2771(request, client as any);

// -----------------------------------------------------------------
// the following is an alternative example using Gelato Fee Oracle, 
// setting maxFee, and calling the incrementFeeCapped(maxFee) method

// populate the relay SDK request body
const requestMaxFee: CallWithSyncFeeERC2771Request = {
  chainId:  BigInt(chainId),
  target: targetContractAddress,
  data: dataMaxFee,
  user: address,
  feeToken: feeToken,
  isRelayContext: true,
};
  
// send relayRequest to Gelato Relay API
const relayResponseMaxFee = await relay.callWithSyncFeeERC2771(requestMaxFee, client as any);
Learn more about Implementation of Non ERC2771 SyncFee Calls here and ERC2771 SyncFee Calls here.