ERC-4337: Bundlers, Paymasters, Entry Point

What you'll learn

• Account Abstraction Without Forking Ethereum
• The UserOperation: A New Kind of Transaction
• The EntryPoint Contract
• Bundlers: The New Mempool
• Paymasters: The Economics of Gasless UX
• Aggregators and Version History

Key terms

ERC-4337

The Ethereum standard for account abstraction without consensus changes, proposed by Vitalik Buterin, Yoav Weiss, Dror Tirosh, and Shahaf Nacson in September 2021 and deployed to mainnet on March 1, 2023. Implements smart accounts through a separate mempool, off-chain bundlers, and the EntryPoint singleton contract.

UserOperation

The pseudo-transaction data structure at the heart of ERC-4337. Contains sender, nonce, callData, gas limits, optional paymaster info, and a signature blob that the sender account knows how to verify. Lives in the alt-mempool, not the regular Ethereum txpool.

EntryPoint

The singleton smart contract that orchestrates ERC-4337. Deploys per version on every supported chain at the same address (e.g., v0.6 at 0x5FF1...2789, v0.7 at 0x0000...a032). Runs the two-phase validate-then-execute lifecycle and settles gas fees to bundlers.

Bundler

An off-chain service that collects UserOperations from the alt-mempool, packs them into a single Ethereum transaction calling EntryPoint.handleOps, and earns the spread between user-paid priority fees and validator tips. Major bundlers in 2024-2025 include Pimlico, Stackup, Alchemy, Biconomy, and Candide.

Paymaster

A smart contract that volunteers to pay gas on behalf of a user in exchange for whatever consideration it chooses (nothing, an ERC-20 token, an off-chain sponsorship signature). Enables gasless transactions, pay-in-token UX (gas in USDC), and token-gated sponsorship.

Alt-mempool

The parallel mempool that ERC-4337 bundlers monitor. Distinct from the regular Ethereum txpool because UserOperations are not transactions at the protocol layer. Governed by ERC-7562 validation rules that prevent ops from interfering with each other.

validateUserOp

The function every ERC-4337 smart account must implement. Called by the EntryPoint during the validation phase to verify the signature, check the nonce, and (if no paymaster) pay the gas deposit. Bounded by ERC-7562 rules about which storage and contracts it may access.

validatePaymasterUserOp

The function a paymaster contract implements to decide whether to sponsor a given UserOperation. Returns a context blob for postOp settlement plus a validity window. The single chokepoint where paymaster business logic (sponsorship eligibility, token pricing) lives.

Aggregator

An optional ERC-4337 contract that combines many UserOperation signatures (typically BLS) into a single combined signature, verified once by the EntryPoint. Reduces gas dramatically when bundles are large; mainly relevant for high-volume L2s and gaming.

EntryPoint v0.7

The April 2024 EntryPoint release deployed at 0x0000000071727De22E5E9d8BAf0edAc6f37da032 on all supported chains. Introduced PackedUserOperation, separated paymaster address from paymaster data, and improved gas accounting. The dominant target for new ERC-4337 accounts in 2024-2025.