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ERC-4337 Overview

A quick overview of the standard for developers.


This page gives a simplified overview of ERC-4337 so that developers can get a basic understanding of the different components and how they can be pieced together to build their applications. For a full run down on the spec we recommend going straight to the source.


There are four main components to ERC-4337: a UserOperation, Bundler, EntryPoint, and Contract Account. These can be supplemented by Paymasters and Aggregators.

Major components of ERC-4337

  • UserOperations are pseudo-transaction objects that are used to execute transactions with contract accounts. These are created by your app.
  • Bundlers are actors that package UserOperations from a mempool and send them to the EntryPoint contract on the blockchain.
  • EntryPoint is a smart contract that handles the verification and execution logic for transactions.
  • Contract Accounts are smart contract accounts owned by a user.
  • Paymasters are optional smart contract accounts that can sponsor transactions for Contract Accounts.
  • Aggregators are optional smart contracts that can validate signatures for Contract Accounts.

All Stackup tools are EIP-4337 compliant and composable with other open-source packages to make handling this logic easy.

Stackup tools:

  1. Bundler receives UserOperations via a JSON RPC Client and submits them to the EntryPoint.
  2. Paymaster API Plug-n-Play APIs to enable gasless transactions in your apps.

Recommended ecosystem tools:

  1. eth-infinitism client SDK helps you build UserOperations in your client.
  2. eth-infinitism contracts provides a library of ERC-4337 smart contracts for common use cases.

The following sections describe the UserOperation, Bundler, EntryPoint, Account Contract, and Paymaster Contract in more detail.


All components of ERC-4337 revolve around a pseudo-transaction object called a UserOperation which is used to execute actions through a smart contract account. This isn't to be mistaken for a regular transaction type.

senderaddressThe address of the smart contract account
nonceuint256Anti-replay protection
initCodebytesCode used to deploy the account if not yet on-chain
callDatabytesData that's passed to the sender for execution
callGasLimituint256Gas limit for execution phase
verificationGasLimituint256Gas limit for verification phase
preVerificationGasuint256Gas to compensate the bundler
maxFeePerGasuint256Similar to EIP-1559 max fee
maxPriorityFeePerGasuint256Similar to EIP-1559 priority fee
paymasterAndDatabytesPaymaster Contract address and any extra data required for verification and execution
signaturebytesUsed to validate a UserOperation along with the nonce during verification


A Bundler is a class of actors that can do several things:

  • Listen in to a UserOperation mempool.
  • Runs simulations.
  • Bundles an array of operations.
  • Relays bundles to the EntryPoint.

The dedicated UserOperation mempool is separate and not to be confused with the regular transaction mempool.


Although a public P2P UserOperation mempool is still a work in progress, the advantages of account abstraction can still be leveraged today by relying on private mempools that anyone can spin up.


The EntryPoint is a contract that acts as a central entity for all ERC-4337 accounts and paymasters. It coordinates the verification and execution of a UserOperation. For this reason, it's important for all implementations of an EntryPoint to be audited and not controllable by any single entity.

ERC-4337 EntryPoint Sequence

The above sequence diagram shows how the EntryPoint handles a batch of UserOperations sent by the Bundler. Essentially there are 2 phases.

  1. Verification loop: Verifies that each UserOperation is valid by checking it with both the Account Contract and the Paymaster Contract.
  2. Execution loop: Sends the callData in each UserOperation to the Account Contract.

The verification loop will also make sure that either the Account Contract or Paymaster Contract can pay the maximum gas cost for each UserOperation. In the execution loop any unused gas fee is refunded to the Account Contract or a function is called on the Paymaster Contract to run any required fee logic.


Create2Factory refers to EIP-2470: Singleton Factory. This is a permission-less contract used to deploy Accounts with the same deterministic address on any chain.

Contract Account

The Contract Account is an end user's account. At minimum it needs to check whether or not it will accept a UserOperation during the verification loop.

Additional features to support other account functions like social recovery and multi-operations can be added here too.


The Aggregator is a smart contract that is trusted to validate signatures for Contract Accounts.


The Paymaster is another contract account that handles any UserOperation with sponsored transactions. It is required to do 2 things:

  1. Check whether or not it will accept a UserOperation during the verification loop.
  2. Run any required fee logic in the execution loop.

An example of a Paymaster logic could be to withdraw a certain amount of ERC-20 tokens from the Contract Account after the UserOperation is executed. This allows for a UX where users can pay for gas in any currency they choose.


Putting it all together:

How ERC-4337 works