An AI service marketplace might ask a relayer to pay blockchain gas so a user can approve a small service order without first acquiring native tokens. ERC-2771 enables a recipient contract to recover an intended signer from a trusted forwarder, but it does not decide whether a particular AI agent should be allowed to buy a service. That difference matters when operators advertise “gasless approval” as if it were risk-free delegation. The practical question is what each signature, contract and off-chain policy actually authorizes. The following analysis separates protocol mechanics from purchase intent and gives a repeatable test plan.

Understand the roles before drawing the workflow

ERC-2771, created in July 2020, describes a recipient contract, a trusted forwarder and the original signer of a meta-transaction. The relayer can carry the signed request and pay gas, while a correctly implemented recipient derives the signer through its trusted-forwarder convention. None of those roles automatically represent the merchant, a consumer, an AI model and an account owner as one identity. Draw four lanes: human principal, AI planner, signing wallet and execution/settlement contracts. Add the off-chain operator if it selects service providers or controls a private API key. Each lane must have a clearly stated ability to propose, approve, execute or cancel. If the team cannot identify who authorized the price and recipient, it should not call the result authorized commerce.

Treat the forwarded request as bounded technical permission

A signed request should identify the exact target, intended method, value where supported, nonce and expiration. A broad wildcard call can become a spending channel, especially if an agent controls the payload after the user approves it. The OpenZeppelin Contracts 5.x meta-transaction documentation describes ERC2771Forwarder fields including from, to, value, gas, nonce, deadline and data. Use these fields as a technical baseline, not a complete commercial agreement. Encode the accepted service, maximum price and expected outputs in a separate mandate that the target contract validates. Otherwise, a technically valid signature may still authorize a different service from the one presented in the AI interface.

Inspect the sender context inside every privileged function

The recipient must distinguish the transaction sender, which can be the forwarder, from the reconstructed original signer. A contract that checks raw msg.sender in one place and an ERC-2771-aware context elsewhere can accidentally grant the forwarder privileges it was never meant to have. Build static analysis rules and unit tests for every authorization path, including upgrades and inherited modules. Review calldata handling carefully: ERC-2771 implementations append the signer address, which changes assumptions about raw calldata length. OpenZeppelin explicitly warns about contracts relying on particular calldata length semantics. Add negative tests for calls directly from an untrusted forwarder and for malformed forwarded payloads. The system should revert before changing balances, royalties or entitlements.

Decide what the AI planner may change after approval

A model may suggest a cheaper provider after the user has already signed a request. That is a new purchase intent, not a harmless routing optimization, if merchant, jurisdiction, cancellation terms or data handling change. Define an approved order object with immutable fields and a small explicitly allowed substitution set. Require renewed human approval for anything outside that envelope. Record which AI-produced recommendation was presented and what structured mandate the user actually signed. Do not rely on a screenshot of a chat bubble as the sole source of truth. If the order includes a data-processing service, rights and retention conditions may matter as much as token value. An on-chain receipt proves an execution event, not that the off-chain service fulfilled its advertised obligations.

Design replay, expiration and gas-abuse tests

Simulate a signed request submitted twice, a nonce reused on a different forwarder and a request delivered just after its deadline. Repeat against the intended chain and a fork with the same contract code to identify missing domain separation. A relayer should refuse unsupported targets, excessive gas commitments and unknown call selectors before spending funds. Monitor fee spikes without automatically raising spending caps. Where a forwarder batches many requests, test whether one invalid request can interfere with unrelated users. Keep the gas-payment policy separate from service pricing: a sponsored transaction does not imply the service itself is free. A robust implementation logs rejected requests and reasons without publishing the user’s full private service parameters.

Separate payment receipt from evidence of delivery

After execution, a blockchain transaction can establish when an order was accepted and which contract handled it. It may not prove that an AI inference returned a useful answer, that a file was delivered or that the buyer could exercise a cancellation right. Design an off-chain acceptance record with a request hash, expected output schema, deadline, dispute route and reviewer identity. Anchor only a minimal commitment on-chain if the business needs public verification; avoid placing raw personal prompts and files in permanent transaction data. A settlement workflow can withhold release pending acceptance or refund, but the escrow conditions must be explicit. This is a business application of smart contracts, not a capability supplied automatically by ERC-2771.

Publish a bounded-approval acceptance suite

Test one authorized order, an altered price, an altered target, a revoked signer, an expired deadline, a duplicated nonce, a failed relayer and a missing service result. For each scenario document the expected contract state, user-visible message and audit evidence. Use a fresh deployment in a test environment and verify that wallet signatures show intelligible amounts and destinations before enabling production. Conduct external review for financial contracts with significant exposure. The launch gate is not “all transactions relay successfully”; it is “an agent cannot use a gasless convenience layer to exceed a human-approved mandate.” That distinction protects both the buyer and legitimate operators building AI service markets on Web3.

Sources

Ethereum ERC-2771, created 1 July 2020; OpenZeppelin Contracts 5.x, Meta-Transactions.