What is Bundler?

A Bundler is a specialised participant in Ethereum account abstraction infrastructure that collects UserOperations from smart accounts, combines them into transactions, and submits them to the ERC-4337 EntryPoint contract for execution. Bundlers make it possible for users to interact with smart accounts without individually submitting conventional Ethereum transactions from externally owned accounts.

The role emerged as part of ERC-4337, which introduced account abstraction without requiring fundamental changes to Ethereum’s consensus layer. Instead of sending ordinary transactions directly into Ethereum’s standard transaction flow, ERC-4337 users create UserOperations. These describe actions that a smart account wants to perform, together with information needed for validation and gas handling.

Bundlers provide the connection between these UserOperations and Ethereum’s normal transaction system. They collect operations, check whether they are suitable for inclusion, package multiple operations into a single transaction, and pay the transaction gas upfront. The costs are then accounted for through the ERC-4337 mechanism.

This makes a Bundler infrastructure rather than a wallet feature. It does not hold the user’s primary private key and does not decide whether a user’s smart account is authorised to perform an action. Its job is to transform valid account abstraction operations into transactions that Ethereum can actually process.

Where Bundlers Fit Into ERC-4337

ERC-4337 separates several responsibilities that are combined in a conventional Ethereum transaction. A smart account contains programmable account logic, the EntryPoint coordinates validation and execution, a Paymaster can sponsor gas, and a Bundler is responsible for getting UserOperations onto Ethereum.

Users typically submit UserOperations to a dedicated UserOperation mempool rather than Ethereum’s conventional transaction mempool. Bundlers monitor this flow and select operations they are willing to process.

A simplified ERC-4337 transaction path looks like this:

1. A smart account or application creates a UserOperation describing the requested action.
2. The UserOperation is submitted to ERC-4337 infrastructure and becomes available to Bundlers.
3. A Bundler performs preliminary checks and simulation to determine whether the operation can be processed successfully.
4. The Bundler selects one or more suitable UserOperations and packages them for submission.
5. It sends an Ethereum transaction that calls the EntryPoint contract.
6. EntryPoint validates the relevant accounts and executes accepted UserOperations.
7. Gas costs are settled through the ERC-4337 accounting mechanism, including Paymaster sponsorship when applicable.

The Bundler therefore acts as an execution intermediary, but it does not replace Ethereum validators or block builders. After the Bundler submits its transaction, that transaction still enters Ethereum’s ordinary block-production process.

This distinction is important because “bundling” in ERC-4337 refers specifically to packaging UserOperations. It should not be confused with constructing an entire Ethereum block or creating MEV bundles for block builders.

UserOperations Instead of Ordinary Transactions

Understanding Bundlers requires distinguishing UserOperations from conventional Ethereum transactions.

A traditional EOA signs and broadcasts an Ethereum transaction. The sender needs sufficient ETH to cover gas, and Ethereum directly validates the transaction according to protocol rules.

ERC-4337 adds another abstraction layer. A UserOperation represents an instruction that should eventually be executed by a smart account, but it is not itself a native Ethereum transaction. It contains fields describing the sender, execution-related data, gas parameters, and information needed for account validation and optional sponsorship.

The Bundler is the participant that eventually places these operations inside an actual Ethereum transaction.

This architecture allows smart accounts to support features that are difficult to implement with conventional EOAs, while still settling execution on Ethereum.

The Bundler’s transaction is the bridge between these two layers of operation.

Why Bundlers Simulate UserOperations

A Bundler cannot safely include every UserOperation it receives. It pays the gas required to submit its Ethereum transaction, so invalid or deliberately malicious operations could create financial losses or denial-of-service problems.

Before including an operation, a Bundler performs validation-related checks and simulation. The objective is to determine whether the operation is likely to satisfy EntryPoint requirements and whether the account or Paymaster can cover the applicable cost.

This is more complicated than simply checking a signature. Smart accounts can contain programmable validation logic, while Paymasters can apply their own sponsorship conditions. Bundlers therefore need to evaluate operations under ERC-4337 rules before committing resources to them.

Simulation also protects the wider UserOperation mempool. Without restrictions, attackers could distribute operations designed to fail only after consuming expensive computation or could manipulate state dependencies so that previously valid operations become unusable.

ERC-4337 defines rules intended to make this environment predictable enough for independent Bundlers to participate without having to trust users, wallets, or Paymasters.

How Bundlers Make Money

Operating a Bundler is an economic activity rather than a purely technical service. The operator pays the gas required to submit the aggregated transaction and expects the ERC-4337 fee mechanism to compensate it.

Bundlers can therefore choose which UserOperations are economically attractive to process. They must consider expected gas usage, offered fees, validation risk, current network conditions, and the possibility that an operation will become invalid before inclusion.

Important factors in Bundler economics include:

• the priority fee or other compensation associated with UserOperations;
• the cost of submitting the final Ethereum transaction;
• the number of operations that can be processed efficiently together;
• simulation and infrastructure costs;
• competition with other Bundlers for attractive UserOperations;
• the reliability of smart accounts and Paymasters involved;
• changes in Ethereum gas prices between operation selection and execution.

Bundling multiple UserOperations can improve operational efficiency because one Ethereum transaction can coordinate the processing of several smart account actions. However, bundling does not make execution free. Every operation still consumes computation and contributes to the transaction’s total gas usage.

A Bundler must therefore balance inclusion volume with expected profitability and execution reliability.

Bundlers and Paymasters Have Different Roles

Bundlers and Paymasters frequently appear together in ERC-4337 discussions, but they solve different problems.

A Paymaster determines whether it is willing to sponsor the gas associated with a particular UserOperation. It can apply conditions such as allowing only specific applications, accounts, tokens, or transaction types.

The Bundler is responsible for delivering eligible UserOperations to EntryPoint. It may process operations funded directly by smart accounts as well as operations sponsored by Paymasters.

This separation allows several independent markets to exist. Wallet developers do not necessarily need to operate their own Bundlers, Bundlers do not need to provide gas sponsorship, and Paymasters can support operations submitted through compatible Bundling infrastructure.

A single company can offer several of these services together, but their protocol functions remain separate. This modularity is important for avoiding unnecessary dependence on one provider.

Bundler Infrastructure and Decentralisation

ERC-4337 was designed so that Bundling does not need to be controlled by one central operator. Multiple Bundlers can independently receive UserOperations and compete to submit them.

This is important for availability and censorship resistance. If a wallet depends entirely on one proprietary Bundler, an outage or policy decision by that provider could temporarily prevent its users from submitting ERC-4337 operations through the normal route.

A competitive Bundler ecosystem gives wallets and applications alternative paths. In principle, users should be able to send operations to other compatible Bundlers if one refuses or fails to process them.

In practice, decentralisation depends on more than the protocol allowing multiple operators. Running reliable Bundler infrastructure requires Ethereum connectivity, transaction simulation, mempool handling, gas estimation, monitoring, and sufficient capital for transaction submission. Differences in implementation and application integrations can also influence which providers receive the largest share of UserOperations.

Bundlers therefore introduce an infrastructure layer whose concentration matters even though they do not control Ethereum consensus.

Bundlers Are Not Ethereum Block Builders

The similarity between the words “Bundler” and “Builder” can create confusion, especially because both roles aggregate blockchain-related data before something is submitted to Ethereum.

Their responsibilities are fundamentally different. A Bundler works specifically with ERC-4337 UserOperations and turns them into a transaction addressed to EntryPoint. A block builder works at the Ethereum block-production level and constructs a complete execution payload containing transactions from many unrelated sources.

The Bundler’s submitted transaction can itself later be selected by a block builder alongside swaps, transfers, contract calls, MEV bundles, and other Ethereum transactions. The resulting block is then proposed through Ethereum’s normal consensus process.

This means the two roles operate at different layers. Bundlers belong to account abstraction infrastructure, while block builders belong to block production infrastructure.

Keeping this distinction clear is particularly important as both systems become more specialised. ERC-4337 can have many independent Bundlers without changing who builds Ethereum blocks, and changes to the builder market do not fundamentally change the purpose of Bundlers.

Why Bundlers Matter for Smart Accounts

Bundlers solve a practical problem created by ERC-4337: UserOperations are useful abstractions for programmable accounts, but Ethereum blocks ultimately contain Ethereum transactions. Someone needs to validate, package, fund, and submit those operations in a form the EntryPoint contract can process.

That participant is the Bundler.

Its role enables smart account features without requiring every user to understand or directly manage the underlying transaction submission process. Combined with programmable smart accounts and optional Paymasters, Bundlers help support gas sponsorship, alternative authentication systems, batched actions, Session Keys, and other account abstraction features.

At the same time, Bundlers do not give users free execution, replace validators, build Ethereum blocks, or control wallet private keys. They occupy a specific layer between smart account requests and Ethereum transaction execution.

As account abstraction expands, reliable and competitive Bundler infrastructure becomes important for both usability and resilience. The more easily wallets can route valid UserOperations through independent Bundlers, the less the account abstraction ecosystem needs to depend on a small number of transaction submission providers.