Build a Paymaster Kit for Gasless Web3 Onboarding

Define the sponsorship scope

Before deploying your paymaster contract, you must explicitly define which transactions will be sponsored. This scope acts as your primary security perimeter. Without strict boundaries, a paymaster becomes an open wallet, vulnerable to rapid draining through spam or malicious contract interactions.

Start by listing the specific contract addresses and function signatures you intend to cover. Limiting sponsorship to known, audited contracts prevents attackers from routing funds through unknown intermediaries. For example, if you are onboarding users to a specific dApp, only sponsor calls to that dApp’s core logic, not its ancillary or unverified utilities.

Next, establish hard limits on gas and value. Set a maximum gas limit per transaction to prevent infinite loop attacks. If your paymaster supports ERC-4337, configure the maxGasLimit and maxFeePerGas in your bundler configuration. These caps ensure that even if a user’s transaction is malformed, the cost to you remains predictable and bounded.

Note: Define strict limits on sponsored actions to prevent wallet draining attacks.

Finally, consider time-based or frequency-based constraints. A paymaster can reject transactions from the same user address if they exceed a certain number of sponsored calls within a specific window. This prevents bots from flooding the network with cheap transactions, ensuring your sponsorship budget goes toward genuine new users rather than automated scripts.

Deploy the paymaster smart contract

This section walks through deploying a compliant ERC-4337 paymaster contract on the target chain. The process involves configuring the contract parameters, deploying via your preferred provider, and verifying the source code on-chain.

Configure Paymaster Parameters

Before deployment, define the operational boundaries of your paymaster. You must specify the trustedForwarder address (the entry point contract) and the gas limits for user operations. Configure the sponsorship policy to determine which user operations qualify for gas payment. This usually involves setting up a simple allowlist or integrating with a reputation oracle to prevent spam. Ensure your contract inherits from the standard PayingPaymaster interface provided by the ERC-4337 specification to maintain compatibility with bundlers.

Deploy the Contract

Use your deployment script (e.g., Hardhat or Foundry) to push the bytecode to the target network. Pass your configuration variables as constructor arguments. During this phase, verify that your wallet has sufficient native gas tokens (ETH, MATIC, etc.) to cover the deployment cost. Once the transaction is confirmed, record the new contract address. This address will serve as the paymasterAddress in your smart account interactions.

Fund the Paymaster

A gasless paymaster cannot operate with an empty balance. Send an initial funding transaction of native tokens to the newly deployed contract address. The amount depends on your expected volume and the gas price of the target chain. You can automate this funding process using a keeper network or a simple off-chain script that monitors the balance and refills it when it drops below a threshold.

Verify Source Code

Verify your contract on a block explorer like Etherscan or BscScan. This step is critical for trust and debugging. It allows bundlers and other developers to inspect the contract logic and ensure it matches the deployed bytecode. Use the --verify flag in Hardhat or the forge verify-contract command in Foundry, providing the constructor arguments used during deployment.

Do I need to deploy a new paymaster for each chain?

How much gas should I fund initially?

Can I use a single paymaster for multiple entry points?

Integrate the bundler and signer

Connecting your paymaster to a bundler and managing signing keys is the core infrastructure work for gasless onboarding. The bundler collects user operations and submits them to the mempool, while the paymaster uses a dedicated signer to cover the gas fees on behalf of the user.

1. Configure the bundler connection

Your paymaster contract must be able to interact with the bundler’s entry point address. This usually involves setting the entry point address as a constant or configuration variable in your smart contract.

The bundler listens for UserOperation objects. When a user initiates a transaction, the front end sends this object to the bundler. The bundler then validates the operation and forwards it to your paymaster for sponsorship verification.

2. Set up the paymaster signer

The paymaster needs a private key to sign the gas payment transactions. This signer is distinct from the user’s wallet and the contract owner’s key.

Store this private key securely using environment variables or a vault service. Never commit this key to your code repository. The signer will be used to sign the signature field in the UserOperation when the paymaster agrees to pay the gas.

3. Implement sponsorship logic

Before signing, the paymaster must verify that the user operation is eligible for gas sponsorship. This logic should check conditions like user identity, transaction type, or account balance.

If the operation passes your checks, the paymaster signs the transaction. If it fails, the paymaster returns a revert reason, and the bundler will not include the operation in the block.

4. Test with a local bundler

Use a local bundler environment like Alchemy’s or a local Hardhat node to test the integration before deploying to mainnet. This allows you to verify that your paymaster correctly signs operations and that the bundler accepts them.

Focus on testing edge cases: invalid signatures, failed sponsorship checks, and network congestion. Ensure your paymaster reverts gracefully when conditions are not met.

Fund the paymaster account

Build a Paymaster Kit for Gasless Web3 Onboarding works best as a sequence, not a scramble through settings. Do the minimum first: confirm compatibility, connect the core hardware, update only when needed, and test the result before adding optional features. That order keeps the task understandable and makes failures easier to isolate. After each step, pause long enough for the interface to finish syncing. Many setup problems are timing problems disguised as configuration problems. If the same step fails twice, record the exact error, restart the smallest affected piece, and retry before moving deeper.

The simplest way to use this section is to keep the setup small, verify each change, and record the stable configuration before adding optional accessories.

Test gasless transactions end-to-end

Verify your Paymaster Kit by simulating a complete user journey. The goal is to confirm that a user can sign a transaction, have the paymaster cover the gas fee, and see the state change on-chain without spending their own ETH.

Deploy test contracts locally

Start your local development node using Hardhat or Foundry. Deploy your ERC-2771-compatible forwarder and the Paymaster contract to localhost:8545. Ensure the Paymaster is funded with ETH to cover gas costs.

Configure the signer and provider

Update your web3 provider to use the local node. Configure your signer to include the PaymasterAndData suffix in the transaction calldata. This tells the relay to route the gas payment through your Paymaster contract.

Send a gasless transaction

Execute a transaction from a user wallet to a target contract. Monitor the node logs. You should see the transaction sent with the Paymaster data attached. Verify that the user’s ETH balance remains unchanged while the Paymaster’s balance decreases.

Validate state changes

Check the blockchain explorer or local logs to confirm the transaction was mined. Verify that the target contract’s state updated correctly. If the transaction failed, check the revert reason; it is often a signature validation error or insufficient Paymaster balance.

Common paymaster implementation errors

Even with a solid architecture, small logic gaps in your paymaster contract can drain funds or break user flows. These mistakes are especially costly in gasless onboarding, where a single failed transaction breaks the trust you’re trying to build.

Invalidating legitimate signatures

Your validator must strictly enforce the signature scheme. If you accept weak elliptic curves or fail to verify the v, r, s components against the user’s address, attackers can forge transactions. Always use standard libraries like ecrecover or ecrecover carefully, and ensure the hash being signed matches the exact EIP-712 struct defined in your smart contract.

Underestimating funding buffers

A paymaster goes down the moment its balance hits zero. If you don’t monitor the ETH or token balance and refuel automatically, users will see "insufficient funds" errors. Set up a threshold alert (e.g., at 10% capacity) and an automated refill mechanism. Never rely on manual top-ups for production environments.

Ignoring gas price volatility

Static gas limits fail during network congestion. If you hardcode the gas limit or don’t account for the paymaster’s own execution cost, transactions will revert. Always calculate the total gas required: userGas + paymasterOverheadGas. Use maxFeePerGas and maxPriorityFeePerGas from the user’s request to ensure your paymaster can cover the base fee without overpaying.

Recommended paymaster tools and kits

Building a gasless onboarding flow requires specific SDKs and infrastructure providers. The following tools are standard for integrating ERC-4337 account abstraction into your application.

Biconomy

Biconomy offers a paymaster-as-a-service solution that simplifies the integration of sponsored transactions. Their SDK handles the complex logic of gas sponsorship, allowing developers to pay fees on behalf of users without managing the underlying smart contract infrastructure directly. This is particularly useful for onboarding new users who do not hold native tokens for gas.

Stackup

Stackup provides a robust paymaster infrastructure specifically designed for ERC-4337 bundlers. Their platform offers pre-built paymaster contracts that can be customized for different sponsorship models, such as fixed gas limits or conditional sponsorship based on user actions. Stackup’s API abstracts the complexity of interacting with the entry point contract, reducing development time.

SafeWallet Paymaster

For projects already using SafeWallet (formerly Gnosis Safe), their official paymaster integration provides a seamless way to enable gasless transactions. This tool allows multisig signers or single signers to sponsor transactions for their users. It is ideal for enterprise or DAO contexts where transaction validity is already managed by a trusted signer.

Thirdweb

Thirdweb’s SDK includes a paymaster module that simplifies the deployment and management of custom paymaster contracts. It provides templates and utilities for creating conditional sponsorship rules, such as allowing free transactions for verified users or limiting sponsorship to specific contract interactions. This reduces the need for custom smart contract development.