Bundler

A Bundler is a network node or service that collects, validates, and packages multiple UserOperations—intents signed by smart contract wallets—into a single, standard Ethereum transaction. This aggregation is the core mechanism that enables account abstraction by allowing users to sponsor gas fees with ERC-20 tokens or have third parties pay for their transactions, without requiring each action to be a separate on-chain transaction. By batching these operations, Bundlers significantly reduce the overall gas costs and blockchain congestion associated with mass adoption of smart accounts.

The technical role of a Bundler involves running a specialized bundler client software that listens for UserOperations from a public mempool or private channels. It performs simulation via the eth_call RPC to ensure each operation will succeed and that the paymaster (if used) will cover the gas fees. After validation, the Bundler creates a bundle transaction, pays the network gas fee in the native currency (e.g., ETH), and submits it to an Ethereum block builder. For this service, Bundlers earn fees from the paymaster or take a small margin from the gas fees paid by users.

Bundlers are a critical piece of infrastructure for the ERC-4337 standard, acting as the bridge between the new user-centric transaction model and the existing Ethereum Virtual Machine (EVM) execution layer. Their operation introduces a new actor in the transaction lifecycle, alongside validators and sequencers. To ensure decentralization and censorship resistance, the ecosystem relies on a permissionless network of competing Bundlers, similar to the model of block builders in Proposer-Builder Separation (PBS). Popular implementations include the Stackup, Alchemy, and Pimlico bundler services, which provide reliable infrastructure for dApp developers.

A bundler is a network node in account abstraction systems like ERC-4337 that collects, simulates, and submits UserOperations to the blockchain. It acts as the primary gateway for smart contract wallets, receiving transaction intents from users, packaging them into a standard Ethereum transaction, and paying the network fees on their behalf. This role is analogous to a relayer in other systems, but with the specific duty of enabling gas abstraction and batch processing for enhanced efficiency and user experience.

The bundler's workflow begins by receiving a UserOperation object from a user's smart contract wallet. This object contains the transaction's intent, signature, and other data, but crucially, it is not a standard Ethereum transaction. The bundler must first run a local simulation using the eth_call RPC method to verify the operation's validity—checking signatures, ensuring the wallet has sufficient funds, and confirming it will not revert. This simulation is essential for the bundler's economic security, as it is responsible for covering the gas costs of failed transactions.

After successful simulation, the bundler proceeds to package one or more validated UserOperation objects into a single, atomic bundle. It then creates and signs a standard Ethereum transaction that calls the system's EntryPoint contract. The EntryPoint is a singleton contract that orchestrates the execution of each user's operation, handling gas payment logic and interacting with the individual smart contract wallets. By submitting this single transaction, the bundler pays the gas fees for the entire bundle upfront, later being reimbursed by the EntryPoint from the users' deposited funds.

For this service, bundlers earn revenue through priority fees (tips) included in the UserOperation and may also collect a small surcharge. They compete in a permissionless marketplace, where users can choose bundlers based on reliability, speed, and cost. This design ensures that no single entity controls access, maintaining the decentralized ethos of the network while providing the critical infrastructure for gasless and seamless user transactions.

The Bundler Process Flow is the deterministic sequence of actions a bundler performs to facilitate a transaction on an ERC-4337 account abstraction stack. It begins with receiving UserOperations from a UserOperation mempool. The bundler validates each operation's signature and paymaster sponsorship logic off-chain, simulating execution to ensure it will succeed and that the signer's account can cover gas fees (or that a paymaster will). This simulation is critical, as the bundler is financially responsible for the cost of failed on-chain transactions.

Following successful simulation, the bundler proceeds to bundle construction. It aggregates the validated UserOperations into a single batch. The bundler then creates and signs a bundle transaction, which is a standard Ethereum transaction whose calldata contains the list of operations. A key responsibility here is managing gas pricing; the bundler must set an appropriate base fee and priority fee (tip) to ensure the bundle is mined in a timely manner, balancing cost against user experience and its own profit margins from priority fees.

The final phase is on-chain execution and post-processing. The bundler submits the bundle transaction to the public mempool or a private relay. Once mined, the EntryPoint smart contract takes over, executing each UserOperation in sequence. It validates paymaster deposits, deducts gas fees, and refunds the bundler for any excess. The bundler monitors this outcome, handling any reverts and ultimately distributing the successful transaction results back to the original users or applications that submitted the UserOperations.