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AA21 didn't pay prefund

The EntryPoint contract reverts with AA21 didn't pay prefund when a smart accountMetaMask smart account A smart contract account that supports programmable behavior, delegated permissions, flexible signing options, and other advanced features. doesn't have enough native token balance to cover the gas cost of the user operationUser operation A signed instruction package that tells a smart account what executions to perform..

Before executing a user operation, the EntryPoint requires the sender account to prefund the expected gas cost. If the account's balance is lower than the required prefund, the EntryPoint reverts the operation.

Solution​

Fund the smart account​

Fund the smart account with enough native tokens to cover the required prefund. Use Viem's estimateUserOperationGas to get the gas estimates from your bundlerBundler A service that collects user operations from smart accounts, packages them, and submits them to the network., then calculate the required prefund based on the EntryPoint version.

import { formatEther } from "viem";

const gasEstimate = await bundlerClient.estimateUserOperationGas({
  account: smartAccount,
  calls: [{ to: "0x...", value: 0n }],
});

const { maxFeePerGas } = await publicClient.estimateFeesPerGas();

const requiredGas =
  gasEstimate.verificationGasLimit +
  gasEstimate.callGasLimit +
  (gasEstimate.paymasterVerificationGasLimit ?? 0n) +
  (gasEstimate.paymasterPostOpGasLimit ?? 0n) +
  gasEstimate.preVerificationGas;

const requiredPrefund = requiredGas * maxFeePerGas;

const balance = await publicClient.getBalance({
  address: smartAccount.address,
});

if (balance < requiredPrefund) {
  console.log(
    `Insufficient balance: account has ${formatEther(balance)} ETH, ` +
    `but needs ${formatEther(requiredPrefund)} ETH`
  );
}

Use a paymaster​

You can use a paymasterPaymaster A service that pays for user operations on behalf of a smart account. to sponsor the gas fees for the smart account, so the account doesn't need to hold native tokens. For more information about configuring a paymaster, see Send a gasless transaction.

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