aa21-prefund.md

description	How to resolve the AA21 didn't pay prefund error when submitting user operations.
sidebar_label	AA21 didn't pay prefund
keywords	AA21 pay prefund user operation troubleshooting

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AA21 didn't pay prefund

The EntryPoint contract reverts with AA21 didn't pay prefund when a smart account doesn't have enough native token balance to cover the gas cost of the user operation.

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 bundler, 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`
  );
}

import { formatEther } from "viem";

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

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

const requiredGas =
  gasEstimate.callGasLimit +
  gasEstimate.verificationGasLimit +
  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 paymaster 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.