Gas Sponsorship with Reown

In this recipe, you will learn how to:

• Create a Paymaster Policy on Cloud
• Use the policy in your dApp to sponsor transactions
• Sponsor using Appkit’s embedded account

This guide takes approximately 20 minutes to complete and is split into 2 parts. First, we will be using pre-made policies that interact with smart accounts deployed in service of Appkit lab and validated there. The second half involves integrating it into your own dApp.

Let’s dive in!

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Prerequisites

• A fundamental understanding of JavaScript and React.
• Obtain a new project Id on Reown Cloud at https://cloud.reown.com
• Since Gas Sponsorship is an early access feature, your projectId needs to be access listed. Please contact us.

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Part 1: Steps to Set up a policy

1. Open the Cloud dashboard and click on “Paymaster” in the sidebar.
2. Click “Add Policy”. When prompted, choose “Testnet” chains for testing.
3. Enter your Policy name and click next.
4. On the chain selection screen, simply click next since all testnet chains are enabled.
5. You’ll then see the Control Sponsorships screen.
6. For most cases, the no-code builder works well, though there is a detailed guide for custom policies if needed.

Required Items:

1. Input the contract address
2. The ABI of the contract
3. Select the method you’ve chosen to sponsor
4. IMPORTANT: If you would like to sponsor the deployment of smart accounts on your app, tick the Sponsor Smart Account Deployment checkbox.
5. Click “Create”.

You now have a functional policy!

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Retrieving the Paymaster URL

Each Paymaster is linked to a specific chain. Click the “RPC URLs” button to view all supported chain URLs.

Select Sepolia and copy the URL provided to get your paymaster URL.

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Testing the Paymaster Out

Before integrating this functionality into your dApp, you can test that you set up the policy by visiting Appkit Lab.

Login with Appkit Email or Social Login (Google, Apple, etc)

Scroll down to “Send Calls with custom abi (Paymaster Service)” and

1. Input your smart contract address
2. Paste the ABI of the contract
3. Select the method you’ve chosen to sponsor
4. Add any method arguments in the format of an array
5. Add any value if necessary
6. Paste your Paymaster URL
7. Hit send calls.

You can check it in two simple ways:

• In your browser’s network tab, filter for sendUserOperation. If the payload includes paymasterAndData, it’s working.
• Alternatively, copy the send call result, paste it into “Get Call Status”, then use the transaction receipt on Sepolia Etherscan. It will show the paymaster covering the gas.

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Part 2: Integrating with your dApp

After setting up your policy, you can seamlessly add blockchain call functionality to your dApp using Wagmi’s useSendCalls hook. This hook simplifies interacting with the blockchain, allowing you to trigger transactions directly from your React component.

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Define the ABI

const donutContractAbi = [
  {
    inputs: [
      {
        internalType: 'uint256',
        name: 'amount',
        type: 'uint256'
      }
    ],
    name: 'purchase',
    outputs: [],
    stateMutability: 'payable',
    type: 'function'
  },
]

const donutContactAddress = '0x2E65BAfA07238666c3b239E94F32DaD3cDD6498D' as const

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Calculate CallData and Prepare the transaction

const purchaseDonutCallData = encodeFunctionData({
  abi: donutContractAbi,
  functionName: 'purchase',
  args: [1]
})

const TEST_TX = {
  to: donutContractaddress,
  value: parseEther('0.0001'),
  data: purchaseDonutCallData
}

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Import sendCalls from Wagmi

import { useSendCalls } from 'wagmi/experimental'

// Inside the Component

const { sendCalls } = useSendCalls({
  mutation: {
    onSuccess: hash => {
      setLoading(false)
      toast({
        title: 'SendCalls Success',
        description: hash,
        type: 'success'
      })
    },
    onError: () => {
      setLoading(false)
      toast({
        title: 'SendCalls Error',
        description: 'Failed to send calls',
        type: 'error'
      })
    }
  }
})

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Add Event Handler to a button

const onSendCalls = useCallback(() => {
  setLoading(true)
  if (!paymasterServiceUrl) {
    throw Error('paymasterServiceUrl not set')
  }
  sendCalls({
    calls: [TEST_TX],
    capabilities: {
      paymasterService: {
        url: paymasterServiceUrl,
        context
      }
    } 
  })
}, [sendCalls, paymasterServiceUrl])

return (
  <Button
    onClick={onSendCalls}
    disabled={!sendCalls}
  >
    SendCalls w/ Paymaster Service
  </Button>
)

For more information on sponsoring calls, read more in our docs.

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Troubleshooting

If you’re running into issues getting a transaction to be successfully sponsored, it’s best to isolate issues. So to get started, first:

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Setup Environment

1. Use AppKit’s Embedded Smart Wallet: Login using social login (Ex. Gmail) or your email

2. Ensure the code performs send calls as simply as possible., eg: hardcode the capabilities object and remove any extra conditional logic.

3. Keep the network tab on your browser console open

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Ensure a call to the paymaster is being made

Since the embedded wallet lives on your browser, you can observe the network calls it makes.

If, after performing the sendCalls and approving it in the AppKit Embedded Wallet, there is no call to the paymaster URL provided, it likely means that the capabilities were not filled in properly.

Ensure the capabilities object you’re sending looks like this:

paymasterService: {
  url: 'https://paymaster-api.reown.com/<CHAIN_ID>/rpc?projectId=<YOUR_PROJECT_ID>',
}

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Paymaster was called but returned an error

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Simulation Errors

The most common errors encountered with integrating Paymasters typically come in the form of simulation errors.

Thankfully, the paymaster provides a full report of user ops that run into errors. For example:

{
  "error": {
    "message": "The UserOp does not satisfy requirements of policy",
    "details": {
      "message": "Could not successfully simulate call.",
      "cause": "Filtered simulation calls do not include a call to a smart contract.",
      "debugInfo": {
        "simulatedTransaction": { /* full simulation information */ },
        "errors": [
          {
            "error": "executionreverted"
          },
          {
            "error": "out of gas"
          }
        ]
      }
    }
  }
}

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No calls were made to the specified contract.

1. Ensure that the chain ID provided in the paymaster URL matches the network you are on

2. Ensure you are targeting the contract and method specified in your policy

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Not sure if everything’s set up correctly?

1. Confirm a call to the Paymaster is being made via the network tab
2. Ensure the response from the call is not 0x or an error.
3. Verify the call to the bundler (also viewed in the network tab) that is performing sendUserOperation has filled the paymasterData field
4. Grab the user op hash (result from sendUserOperation) and paste it into a block explorer that supports searching by User Op Hashes like BlockScout
5. Ensure the paymaster sponsor field is filled

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Links

• Reown Cloud
• Appkit lab