Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly/idempotent/non-destructive, and the description goes well beyond that: it states the check is fully offline, requires no RPC, describes the exact validation steps (domain rebuild, signer recovery, payTo/amount/asset binding, validAfter/validBefore window), and enumerates what it deliberately does not verify. That is genuinely useful behavioral context for trusting the result.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences, front-loaded with the scope, then the validation logic, then the exclusions. Every clause carries information an agent needs and nothing is padded.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With an output schema present, the description need not document return values; it covers the input contract, the verification semantics, and the boundaries of what a positive result means. Nothing material is missing for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3, but the description adds real meaning by explaining how 'requirement' fields are consumed (extra.name/extra.version/asset/chain id from eip155 are used to rebuild the EIP-712 domain) and by noting that 'payload' can be an object or a raw Base64 PAYMENT-SIGNATURE header. It does not explain the 'now' override beyond the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description names a precise verb+resource ('Offline cryptographic check of an x402 exact EVM payment authorization') and pins the standard (EIP-3009 TransferWithAuthorization, EIP-712). This cleanly separates it from the EVM-agnostic and Solana siblings (verify_solana_transaction, verify_attestation) without needing to open a schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives strong context: use this for offline signature verification with no RPC, and it explicitly excludes balance, nonce/replay and settlement checks, which tells the agent when NOT to rely on it. It stops short of naming an alternative tool to use for those excluded concerns, so it is not a full when/when-not/alternative statement.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.