Bundler

A bundler is an infrastructure node in the ERC-4337 account abstraction system. When a user with a smart account wants to execute a transaction, they sign a UserOperation (UserOp) — a structured intent object — rather than a standard Ethereum transaction. Because UserOps are not standard L1 transactions, they cannot be directly submitted to Ethereum by the user. Instead, a bundler collects pending UserOps from the alternative mempool, validates them, packages multiple UserOps together, and submits a single bundled transaction to the EntryPoint contract. The bundler pays ETH gas on-chain and is reimbursed by the accounts or paymasters involved in the bundled operations.

How Bundlers Work

The following sections cover this in detail.

Step-by-Step Flow

User signs a UserOperation — specifying target contract, calldata, gas limits, paymaster (optional), signature, and nonce — using their smart account’s signing key
UserOp is sent to the ERC-4337 mempool — a separate peer-to-peer network of bundlers (or directly to a specific bundler’s private RPC endpoint)
Bundler validates the UserOp — simulating validateUserOp on the smart account (and validatePaymasterUserOp if there’s a paymaster) to check it will succeed without reverting, won’t drain gas unexpectedly, and meets the bundler’s profitability threshold
Bundler groups UserOps — combines multiple validated UserOps into one Ethereum transaction calling handleOps() on the EntryPoint
EntryPoint executes each UserOp sequentially — calling the smart account’s validateUserOp then execute, and interacting with the paymaster if present
Bundler receives compensation — from the gas refunds built into the EntryPoint (each account or paymaster reimburses the bundler for gas + a priority fee)

The Alternative Mempool

The ERC-4337 UserOp mempool is separate from Ethereum’s standard pending transaction mempool. Bundlers peer with each other and share UserOps before inclusion. This keeps the EVM mempool clean (no UserOps that can’t be directly executed by validators) while allowing bundlers to prioritize profitable bundles.

Validation Restrictions (ERC-7562)

To prevent griefing and DoS attacks, bundlers enforce strict simulation rules (formalized in ERC-7562):

validateUserOp may not access banned opcodes (e.g., SELFBALANCE, TIMESTAMP, BLOCKHASH) — their outputs depend on block context and could cause simulation≠execution mismatches
validateUserOp may not access storage not associated with the account or paymaster — prevents one UserOp’s validation from depending on state another UserOp will modify
Violation of these rules = bundler rejects the UserOp as “simulatable but may fail on-chain”

These restrictions make smart account and paymaster development more constrained than regular Solidity — they’re a common source of bugs.

Bundler Economics

Bundlers are rational economic actors. They submit bundles only when:

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ΣUserOp gas reimbursements > L1 gas cost of handleOps() call + overhead

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Priority fee incentive: UserOps specify a maxPriorityFeePerGas. Bundlers prefer UserOps with higher priority fees, similar to how validators prefer transactions with higher tips.

MEV opportunity: Bundlers can also extract MEV from UserOp ordering within a bundle — e.g., placing arbitrage UserOps favorably. This makes bundling analogous to block building in the PBS ecosystem.

Private vs. public mempools: Many DApps send UserOps to specific bundler endpoints (Biconomy, Pimlico, Alchemy) rather than the public mempool, for latency and reliability. This creates some centralization pressure.

Major Bundler Providers

Provider	Notes
Pimlico	Most widely used public bundler; supports EntryPoint v0.6 and v0.7; `permissionless.js` client
Alchemy	Bundler + paymaster via Rundler (Rust implementation)
Stackup	Early bundler with open-source implementation
Etherspot	Skandha bundler (TypeScript), multi-chain
Biconomy	Managed bundler bundled with their paymaster service

The ERC-4337 association maintains canonical bundler specs. Rundler (Alchemy/Rust) and Skandha (Etherspot/TS) are the two most production-deployed open-source implementations.

Bundler vs. Sequencer vs. Validator

Role	What It Does
Bundler	Collects UserOps, submits bundled tx to EntryPoint
Sequencer	Orders transactions in an L2 rollup
Validator	Proposes/attests Ethereum L1 blocks

Bundlers run on top of existing Ethereum infrastructure — they’re not block producers. A bundler submits a regular Ethereum transaction (just one containing bundled UserOps), which is included by validators via the standard block auction.

History

2021 — ERC-4337 proposed by Vitalik Buterin, Yoav Weiss, Dror Tirosh, Shahaf Nacson, Alex Forshtat, Kristof Gazso, and Tjaden Hess. Bundlers specified as the gas relay layer.
2023 — EntryPoint v0.6 deployed and audited by OpenZeppelin. First production bundlers launch (Stackup, Biconomy).
2023 — ERC-7562 formalized the bundler simulation validation rules (banned opcodes, storage restrictions).
2024 — EntryPoint v0.7 improves gas efficiency; major bundler providers migrate.
2024–2025 — Pimlico’s permissionless.js becomes the dominant bundler client SDK; bundler infrastructure commoditizes with competitive pricing.

Common Misconceptions

“Bundlers validate that transactions will succeed.”

Bundlers validate that UserOps should succeed based on simulation, but simulation is not execution. Edge cases (oracle updates, state changes between simulation and execution) can cause on-chain reverts that bundlers absorb the gas cost of — which is why bundlers are careful about simulation rules.

“There’s one global bundler.”

There are many bundlers, both public (any bundler can include your UserOp) and private (you send directly to a specific service). Most production apps use specific bundler providers for latency guarantees.

Social Media Sentiment

Bundlers are primarily discussed in developer-focused communities (ethresear.ch, r/ethdev, ERC-4337 Discord) rather than general crypto Twitter. The discussion centers on: bundler centralization concerns (most UserOps go through a handful of providers), MEV extraction by bundlers, and the operational complexity of running compliant bundlers given ERC-7562 simulation rules. As ERC-4337 adoption grows, bundler infrastructure is increasingly treated as a commodity service rather than a differentiator.

Last updated: 2026-04