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Gasless onboarding is attractive for AI-agent products: a user should be able to try a service before acquiring native tokens. ERC-4337 paymasters support gas sponsorship, but every subsidy introduces a financial control problem. A wallet becoming easier to use is not evidence that transaction risk has become easier to govern.
What the paymaster actually promises
ERC-4337 documentation describes a paymaster as a contract that can pay gas for a UserOperation under programmable conditions. The EntryPoint validates the operation and draws from a deposit; the sponsor therefore faces both user-experience and treasury consequences. Sponsorship is not equivalent to authorizing the underlying business transaction. An AI agent may have permission to prepare a service order without permission to spend a subscription budget or transfer assets. Design two separate checks: does the account authorize this call, and is this call eligible for someone else to fund its gas? A signature can establish an account-side authorization path, but the sponsor still needs its own budget, abuse controls and contract allowlist. If the transaction fails after inclusion, gas has still been consumed. This is the crucial economic difference from a conventional free-trial button.
Choose a sponsor policy that is auditable
Start with a narrow allowlist of contract addresses, permitted selectors, supported chains, maximum gas exposure per operation and an expiry on any sponsorship voucher. Bind a voucher to a user, intended destination, nonce or equivalent replay-resistant identifier, and a policy version. Avoid signing an unbounded blanket sponsorship for arbitrary target contracts. Before the bundler accepts the operation, the paymaster validation logic must fit the relevant deterministic execution constraints. Its policy should not rely on live calls to off-chain identity services inside execution. Instead, evaluate customer eligibility off-chain when issuing a short-lived approval and independently verify the resulting cryptographic evidence on-chain. An AI assistant may recommend an action, but there must still be an explicit human-approved spending scope or a previously approved narrow automation policy.
Model worst-case loss before launch
A paymaster’s true budget is not the nominal number of sponsored transactions multiplied by average gas. Model the maximum gas commitment for valid operations, a margin for reverted executions, and any deposit required for continuity. For a hypothetical cohort of 5,000 activated accounts, suppose 20 percent issue one eligible operation per day and the sponsor caps each operation at a declared monetary equivalent of $0.12. The daily ceiling for that scenario would be $120 before retry and chain-price uncertainty; it is not an observed cost forecast. Add a global rolling cap, per-account cap, per-operation ceiling and a pause mechanism triggered by abnormal failure rates. Simulate a sharp gas-price change and a coordinated attack from many newly created accounts. A feature designed to reduce onboarding friction must not become an unlimited charge account for adversaries.
Handle bundler simulation and reputation deliberately
ERC-4337 uses bundlers to simulate operations and assess whether they satisfy the protocol’s validation rules. Paymaster documentation also highlights griefing and denial-of-service risks. Treat a failed simulation as an observable outcome with a reason category, rather than asking the AI agent to repeat the same operation blindly. Retries should be bounded, and repeated mismatches between approval data and call data should trigger inspection. Consider a reputation control for newly observed accounts, but do not assume that a new wallet necessarily means malicious behavior. Legitimate customers can be new to self-custody. Prefer evidence linked to the exact sponsored action and merchant relationship over broad profiling. Record which paymaster policy, deposit state and signer key authorized each attempt, without publishing sensitive user data to the public analytics dashboard.
Design refunds and failed services off-chain first
Gas sponsorship should not be confused with escrow or a promise that the purchased AI service succeeded. The blockchain may execute a token transfer while an off-chain video-generation job fails later. Keep an application order ID that bridges the smart-contract receipt and the service outcome. If the business contract promises a refund for non-delivery, use a separate policy for refund amount, authorization and settlement. A sponsorship cost is typically an acquisition or service expense; it is not automatically refundable merely because the user’s output was unsatisfactory. Build a ledger with transaction hash, order identity, sponsored gas, completed service units and any subsequent refund. This structure prevents marketing teams from reporting zero-cost onboarding while hidden gas liabilities grow outside normal product revenue reporting.
Run red-team tests before increasing coverage
The acceptance matrix should include replayed vouchers, high-gas operations that revert, unauthorized contract selectors, chain ID mismatches, exhausted global budgets, concurrent vouchers from the same account and valid sponsorship after a planned signer rotation. Test how fast the emergency pause takes effect and whether its invocation disrupts already funded legitimate operations. Introduce an independent daily reconciliation between EntryPoint deposit changes and the internal sponsorship ledger. A useful launch metric is cost per successful authorized action, not cost per submitted transaction. Include both denied legitimate operations and sponsored failed operations in the dashboard. The best paymaster design offers predictable, explainable access to a defined AI service, not limitless transaction execution in the name of user experience.
