In crypto, a bundler is a service in account abstraction (ERC-4337) that collects UserOperations from smart account users, checks them, and submits them to the blockchain bundled into one transaction. Smart accounts can't send transactions themselves the way a normal wallet does; the bundler takes care of that, pays the gas up front and is reimbursed by the accounts or their paymaster. Bundlers are to ERC-4337 what block builders and validators are to normal transactions.
How does a bundler work?
- Receive: wallets and apps send UserOperations to the bundler's RPC endpoint (methods such as
eth_sendUserOperation) or to the shared alternative mempool. - Simulate: the bundler simulates each UserOperation against the EntryPoint contract to check that the signature, nonce and gas payment are valid, and applies the ERC-4337 validation rules that prevent operations from failing later.
- Bundle: valid operations are grouped into one
handleOpscall to the EntryPoint contract. - Submit: the bundler sends that transaction to the network and pays the gas.
- Get reimbursed: the EntryPoint contract pays the bundler from the smart accounts or the paymaster's deposit, including a margin.
Want users to transact without holding gas? See gasless transactions for every method, from paymaster sponsorship and EIP-7702 to relayers and permit signatures.
Why bundlers matter
Without bundlers, there is no ERC-4337: they are the bridge between UserOperations and the blockchain. A good bundler determines how quickly and reliably operations are included, how accurately gas is estimated, and whether operations get stuck. Bundlers also protect themselves: an operation that passes simulation but fails on-chain would cost them gas, which is why the validation rules and the staking of paymasters and factories exist.
Using a bundler: hosted or your own
Most apps use a hosted bundler from an infrastructure provider via an RPC URL. Running your own bundler gives control over costs, latency, supported chains and censorship resistance, and is relevant for wallet providers, high-volume apps and chains without good bundler support. It does require reliable nodes, careful gas pricing and monitoring. See our bundler development services, ERC-4337 implementation and what is gas.
Frequently Asked Questions about Bundlers
Is a crypto bundler the same as a JavaScript bundler?
No. In web development, a bundler (such as webpack, Vite or esbuild) combines JavaScript files into one package for the browser. In crypto, a bundler is the ERC-4337 service that bundles UserOperations into blockchain transactions.
How does a bundler earn money?
UserOperations pay a little more gas than the bundler actually spends; the difference, plus any priority fee, is the bundler's margin.
What is the difference between a bundler and a paymaster?
The bundler submits UserOperations to the blockchain and pays the gas up front. The paymaster is an optional contract that reimburses that gas on behalf of the user.
Can anyone run a bundler?
Yes. ERC-4337 is permissionless: anyone can run a bundler, which prevents a single party from censoring UserOperations.