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AI agents that propose blockchain purchases should not make a wallet signature feel like an opaque confirmation button. EIP-712 provides typed structured messages and a domain separator, but the standard explicitly does not include replay protection. This tutorial shows how a product team can separate a user’s explicit intent, what the wallet displays, what the contract validates and what an application must record after execution.
Translate an AI proposal into a bounded authorization
Consider an AI assistant that suggests a digital asset purchase. The assistant’s proposal is not yet a transaction and should not be treated as independent permission to spend. A signed message needs a specific action, asset identifier, maximum price, destination, expiry and intended chain. The user’s interface must present these fields before a signature is requested. A vague instruction such as ‘buy similar items whenever convenient’ does not translate safely into an unlimited transfer approval. The most reliable design begins with a single, bounded execution request that a user can inspect and decline.
Separate generation from authorization. The model can prepare an explanation, but server-side deterministic code should construct and validate the typed message. Store the canonical request, prompt version, pricing snapshot and human approval event. If the quote expires, require a fresh evaluation rather than extending a signed request invisibly.
Use EIP-712’s domain as an anti-confusion boundary
The EIP-712 specification, created September 12, 2017, describes typed data, domain separation and fields such as name, version, chainId and verifyingContract. These fields can help a wallet and application establish which contract a signature concerns. They are not a license to sign any domain supplied by a website. Compare the chain ID against the active network and the verifying contract against an approved deployment registry before a signature dialog appears.
Document how domain versions change with upgrades. For a multi-chain AI agent, the same contract-like address string must not be assumed to represent the same business contract on every chain. Include the full chain-and-contract tuple in approval logs; do not depend on an address abbreviation in a screenshot.
Implement explicit replay protection outside the signing standard
EIP-712 states that it does not include replay protection. A valid signature can be presented again unless the application’s protocol prevents repetition. Design a nonce per signer, a deadline, an order hash and a consumed-state check. Prefer a single atomic state transition so two parallel submissions cannot both pass a naive ‘not seen yet’ check. An idempotent server retry should return the existing transaction result instead of creating another economic action.
Test a duplicate request before mining, a duplicate after confirmation, two identical orders with different nonces, a copied signature on a second network and a signature submitted by an unexpected relayer. Each test should have a documented expected state. Counting only successful happy-path transactions is inadequate for a system where an AI agent may retry automatically.
Distinguish a readable wallet message from an honest user interface
Structured fields make review possible, but an interface can still mislead. A wallet might summarize only the first few fields while the product’s approval screen shows a friendly description generated by the AI. The description should never replace the exact amount, token, recipient and expiry. Present estimated gas separately from spending limits, and identify whether an operation authorizes a transfer today or grants a permission that could be used later.
For users who cannot interpret contract hexadecimal, show a verified human-readable label while allowing the exact address to be inspected or copied. Unknown contracts should be labeled unknown rather than confidently named by a language model. Accessibility testing should confirm that screen readers announce all material financial terms in a logical order.
Check domain retrieval and verification results
ERC-5267, created July 14, 2022, defines a mechanism for retrieving EIP-712 domain fields from supporting contracts. Where implemented, this can reduce hard-coded assumptions. It does not eliminate the need to validate those fields against the user’s selected network or the product’s known contract identity. The standard notes that a contract may advertise a verifying contract different from itself, so client policy matters.
Build a verification fixture that retrieves the domain, constructs the typed message, signs with a test account and checks the result against a known on-chain verifier. Repeat after a contract upgrade or network migration. Record the field values and contract bytecode version used in the test so failures can be reproduced.
Ship with a clear incident and revocation workflow
A strong release gate covers mismatched domains, expired approvals, reused nonces, manipulated price fields, unknown contracts, account switching and failed chain changes. Set a small initial transaction limit and require human confirmation for every materially new approval scope. Do not let an agent infer permission from a prior chat message or an old wallet connection. Disable the agent’s execution path quickly if a verifier or relayer behaves unexpectedly.
Keep receipts that link the user’s approved message hash, the contract call, transaction status and final asset movement without retaining private conversations unnecessarily. A refund or failed execution is a separate business event from signature validity. The design objective is a user-understandable, auditable authorization boundary, not faster click-through at any cost.
