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AI wallet agents need a human-readable payment budget and a separate audit of the permissions that exist on-chain. This guide maps ERC-20 allowances to task-level consent and highlights the residual authority that can survive key rotation.

Why an AI agent changes the approval threat model

An autonomous assistant can prepare a token payment faster than a person can read it. That convenience creates a specific failure mode: a user approves a third-party spender once, assumes the task is finished, and later discovers that the remaining allowance has not been removed. ERC-20 describes an allowance interface that permits a spender to withdraw tokens through transferFrom, and the interface itself does not know whether the user intended a single AI task or a continuing delegation. This article distinguishes a model’s suggested action, the wallet’s actual authorized action, and the token contract’s allowance state. Those must not be flattened into one “agent approved” label. The result is a review method for Web3 builders rather than a claim that ERC-20 is defective.

Separate task budget, token allowance and signer privilege

Model three different ceilings. A task budget is what the user asked the assistant to spend on an order. The ERC-20 allowance is what the token contract will let a spender transfer. Signer privilege determines which wallet operations can be authorized at all. A task budget of ten units offers no protection if an external contract still holds a much larger approval. Record the token address, chain, spender address, raw allowance, decimals, expiry policy if any, and task identifier before the action. Not every ERC-20 token implements optional metadata in precisely the way a dashboard assumes; the specification makes name, symbol and decimals optional. Use explicit asset identifiers in approvals and avoid relying on human-readable tickers as unique security identifiers.

Build an approval screen with concrete consequences

An approval interface should show the actual spender address, asset, maximum transferable quantity and whether it is a one-time or ongoing permission. For very large or unlimited allowances, explain the risk in normal language instead of hiding it behind a gas-saving promise. Provide a simulation summary separately from the on-chain signed operation. Do not assert that a transaction is safe just because its simulation passed at one block height. An assistant may recommend a transaction, but the human confirmation should cover the precise contract operation. Include an “inspect existing permissions” path that does not require the user to start a new payment. When an ERC-20 allowance changes, refresh its state from the chain and handle false returns where applicable rather than assuming every call behaves identically.

Model rotation as an event, not as retroactive cleanup

Rotating an AI agent’s session key or upgrading a wallet’s execution logic does not automatically erase allowances previously granted to external spenders. The wallet should inventory outstanding approvals before and after a credential change, identify approvals that no longer serve active tasks, and offer a deliberately signed revocation procedure. Be precise: setting an allowance to zero is a token-contract transaction and can require gas or other supported payment mechanics. Network congestion, token-specific behavior and pending transactions can make cleanup non-atomic. The application should therefore display three states: revocation requested, transaction included, and allowance verified after inclusion. An old allowance remaining open after an agent is disabled is a distinct security incident to test.

Distinguish typed signatures from replay protection

Typed structured-data signatures can make a human approval easier to interpret. EIP-712 standardizes structured hashing and domain separation, but its specification explicitly says it does not include replay protection. Designers need concrete nonces, appropriate deadlines, chain and verifying-contract boundaries, and checks that are enforced by the consuming contract. A readable prompt is not the same thing as an enforceable limit. An AI agent should never transform a signature request for a narrow task into a generic perpetual approval without a fresh decision. Test malicious variations in the verifying contract and chain identifier, as well as expired or previously used requests. Keep any agent-generated explanation distinct from the raw signed fields so auditors can compare the two.

A reproducible six-test release gate

Create a fixture wallet holding only test assets and define six cases: exact-value approval; oversized approval refusal; task cancellation with allowance remaining; signer rotation; replayed typed signature; and token metadata mismatch. For each case preserve transaction hash, chain, contract address, intended action, confirmed allowance and displayed explanation. Fail the release if the user sees “revoked” before the chain state supports that claim. Track successful task completion separately from excess allowance exposure, since a convenient workflow can still leave latent permissions. The two primary specifications used here are ERC-20, created in November 2015, and EIP-712, created in September 2017. They describe technical primitives; the task-scoped review matrix and residual-approval audit are original operational proposals. They are not financial advice, and they do not replace smart-contract security reviews.

Sources

Unsplash image by Markus Stickling, illustrative only.