Crypto Gloss

Bundler (Account Abstraction)

Definition:

In the ERC-4337 Account Abstraction stack, a Bundler is an offchain node that listens to an alternative mempool for UserOperations (ERC-4337’s transaction-like objects), validates that each operation is correctly formed and economically viable, bundles multiple UserOperations together, and submits them to Ethereum in a single standard transaction via the EntryPoint contract — earning the gas fees from all bundled operations in exchange for the MEV and coordination work of efficient bundling. Bundlers are the infrastructure backbone that makes ERC-4337’s “smart contract as user account” model viable without requiring Ethereum protocol changes.

ERC-4337 Background

ERC-4337 implements account abstraction without a hard fork by:

  • Introducing UserOperations (pseudo-transactions for smart contract wallets)
  • A separate alt mempool where UserOperations are gossiped
  • An EntryPoint contract that validates and executes batches of UserOperations
  • Bundlers that select operations from the alt mempool and submit them

The entire flow happens within existing Ethereum infrastructure — no consensus layer changes needed.

What a Bundler Does

Step 1 — Listen to the UserOperation mempool:

Bundlers run a node that connects to the ERC-4337 alt mempool (a peer-to-peer network of nodes passing UserOperations).

Step 2 — Validate UserOperations:

Before including a UserOperation, the Bundler simulates it against the EntryPoint to verify:

  • The smart wallet’s signature is valid
  • The wallet has enough ETH (or a Paymaster will cover fees)
  • The operation’s gas limits are sufficient
  • The wallet’s validateUserOp function doesn’t revert

Step 3 — Bundle and Submit:

The Bundler groups multiple valid UserOperations into a single Ethereum transaction calling EntryPoint.handleOps([op1, op2, ...]). This bundles all operations into one on-chain transaction — gas-efficient for the network.

Step 4 — Collect Fees:

The EntryPoint compensates the Bundler with the aggregated gas fees from all UserOperations in the bundle. Bundlers are economically incentivized to include operations with sufficient gas payments.

Bundler Architecture

“`

User Wallet → UserOperation → Alt Mempool

↓

Bundler (validates)

↓

Ethereum TX: EntryPoint.handleOps([op1, op2, op3])

↓

EntryPoint Contract (on-chain)

→ Validates each op

→ Executes each op

→ Pays Bundler

“`

Bundler vs. Block Builder

Feature Standard Block Builder ERC-4337 Bundler
What it processes Standard Ethereum transactions UserOperations (ERC-4337)
Submission target Block proposer EntryPoint.handleOps()
Revenue MEV + priority fees Gas fees from UserOps
Validation Standard ECDSA Smart wallet’s validateUserOp
Mempool Standard mempool Alt mempool (ERC-4337 specific)

Major Bundler Providers

Provider Notes
Pimlico Most widely used; free RPC, open-source reference implementation
Stackup Major bundler with REST API
Biconomy Bundler as part of full AA stack
Alchemy Bundler via Account Kit
Candide Open-source bundler (voltaire)
Etherspot Multi-chain bundler

Bundler Economics and MEV

Bundlers face a nuanced MEV landscape:

  • They can order UserOperations within their bundle, creating intra-bundle MEV opportunities
  • They submit via standard Ethereum transactions, competing for block space via priority fees
  • Some Bundlers integrate with MEV-Boost (the standard Ethereum PBS system) to capture cross-bundle MEV

The ERC-4337 spec explicitly prohibits certain MEV-extracting behaviors (like front-running within the alt mempool) to protect users — Bundlers that violate these rules are ejected from the alt mempool by peers.

Receiving End: EntryPoint Contract

The EntryPoint is a singleton contract (one address on each chain) that:

  • Receives the bundle from Bundler
  • Calls validateUserOp on each smart wallet
  • If validation passes, calls execute on each smart wallet
  • Interacts with Paymasters to cover gas if configured
  • Compensates the Bundler

The EntryPoint v0.6 and v0.7 are the current canonical versions, audited and widely deployed.

Related Terms

Sources

Last updated: 2026-04