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

Annotations only declare readOnlyHint and openWorldHint; the description adds substantial behavior beyond that, including the per-call cost, the exact failure mode when unpaid, the three ways to supply payment, and the fact that normalized values are returned. This is precisely the kind of operational context an agent needs before invoking a paid endpoint.

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?

Front-loaded with the action and the batch limit, then the supported types, then pricing and the payment retry path. Every sentence carries information, though the dense type enumeration is a long clause that could be broken up for faster scanning.

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?

For a single top-level array parameter with no output schema, the description covers the input shape, the cost, and the primary failure/retry loop. It states that normalized values are returned but does not describe the result shape (per-item valid flag, error reason), which is a minor omission given no output schema exists.

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 description coverage is 0%, so the description carries the full burden. It compensates well by explaining that `type` is optional and auto-detected, and by enumerating the accepted type families (IBAN, VAT, EAN/UPC/ISBN, card, EVM, Solana, email, E.164), which effectively supplies a value domain the schema lacks. The `value` field format constraints per type remain implicit.

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 opens with a specific verb-plus-resource ('POST up to 50 identifiers. Validates IBAN...'), then enumerates the exact identifier families handled and the algorithm used for each (mod 97, Luhn, EIP-55). This clearly distinguishes it from narrower siblings such as gtin_validate or ofac_crypto_screen, which cover only a subset of what is listed here.

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 concrete operating context: batch ceiling of 50, type auto-detection when omitted, and $0.005 USDC per call via x402. The payment failure path is spelled out ('returns the x402 payment requirements; retry with an X-PAYMENT or PAYMENT-SIGNATURE header, or params._meta["x402/payment"]'). It does not explicitly say when to prefer a narrower sibling like gtin_validate over this broader validator, which is the only gap.

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