paymaster

A paymaster is an ERC-4337 contract that agrees to pay the gas for someone else's operation. It removes one of the most awkward onboarding hurdles in crypto: normally you cannot do anything on-chain — not even claim an airdrop or make your first swap — until you have first acquired the network's native token to pay gas. A paymaster lets the gas be sponsored, so a user with zero ETH can still transact.

Two patterns dominate. A verifying (sponsoring) paymaster pays the fee outright on behalf of the user — an app might cover gas for new users, a campaign, or all interactions with its contracts — typically gating who it sponsors by checking a signature or an allowlist so it is not drained by strangers. A token paymaster instead lets the user pay in an ERC-20 such as USDC: the paymaster fronts the ETH gas to the network and, usually in a post-operation step, pulls the equivalent value plus a small markup from the user's token balance.

In the ERC-4337 flow the paymaster is woven into validation. During the bundler's simulation the paymaster's validatePaymasterUserOp runs to decide whether it will sponsor this operation, and the EntryPoint requires it to have staked and deposited funds so the gas is guaranteed to be covered before the bundler includes it. An optional postOp callback then settles token payment after execution. The result is a flexible fee abstraction — gasless UX, pay-in-any-token, subscription models — at the cost of trusting the paymaster's solvency and policy, and of extra gas and complexity in every sponsored operation.