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ERC-4337 UserOp/Bundler/EntryPoint Flow Explained

Confidence: Certain Updated 2026-05-26 Review by 2026-09-22 Sources 1 Machine-translated Original (JA)
#systems#wallet#aa#erc-4337#userop#bundler
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Wiki route

This entry sits under systems index. Read it against ERC-4337 Overview · Application-Layer Implementation of Account Abstraction for peer / contrast context and fintech index for the broader system / regulatory boundary.

Key facts

  • A UserOp is not a transaction; it is a sub-unit packaged by a Bundler
  • A Bundler is similar to a block builder, but it operates in the UserOp mempool
  • EntryPoint is a singleton contract and the entry point that every UserOp must pass through
  • Paymaster is an optional contract that either sponsors gas or accepts gas payment in ERC-20
  • Aggregator is optional and batch-verifies multiple signatures(BLS / others)

Mechanism / How it works

End-to-end flow(typical USDC gas-sponsorship scenario):

  1. The user builds a UserOp:the user(or dApp) generates a UserOperation object:

    • sender(SCW address)
    • nonce
    • callData(specific call, e.g., USDC.transfer)
    • callGasLimit / verificationGasLimit / preVerificationGas
    • maxFeePerGas / maxPriorityFeePerGas
    • paymasterAndData(when using a Paymaster)
    • signature(signature of the SCW owner)

  2. Send the UserOp to the alt-mempool:the user sends it through the Bundler RPC(eth_sendUserOperation)

  3. Bundler simulation + packaging:the Bundler forks state locally and executes the UserOp, confirms that the gas estimate is correct, then packages N UserOps into a standard transaction 1 件 and calls EntryPoint.handleOps()

  4. EntryPoint executes:

    • Calls the SCW factory and deploys the SCW(if the SCW does not yet exist)
    • Calls SCW.validateUserOp() to verify the signature + nonce
    • Calls Paymaster.validatePaymasterUserOp() to verify the Paymaster’s sponsorship consent
    • Executes callData(the main business logic)
    • Settles gas:collects from the Paymaster / pays the Bundler

  5. On-chain result:the user can confirm the SCW execution result and does not need to hold ETH

Core innovation point:because a UserOp is not a transaction, Bundlers can perform “batch-processing optimization” — packaging 10 件 UserOps into 1 件 transactions and spreading the on-chain base cost.

Origin & evolution

The UserOp structure went through multiple iterations during the 4337 draft period(2021-2022). Early versions had more fields(wallet, separate initCode, etc.), but were later simplified into the current form. The main simplifications from v0.6(2023-03 public version) to v0.7(2026) were:

  • Integrating initCode into the sender generation logic
  • Splitting Paymaster fields out of calldata and making them explicit
  • Reducing gas overhead by 20-30%

The Bundler economic model introduced a priority-fee auction mechanism after v0.7 , mitigating the early problem that “Bundlers are hard to monetize.”

Sources