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

With no annotations present, the description carries the full burden, and it does disclose the return semantics ('returns true for valid EANs') plus a payment/trial constraint ('5 free trial calls per signed wallet'). It does not say what happens for invalid input (false? error?), what happens with malformed or wrong-length values, or whether a wallet signature is required for paid calls beyond the trial.

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 short clauses, front-loaded with the purpose and the accepted formats, with no filler sentences. The 'Provide value' clause is wasted words, but overall the structure is tight and scannable.

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 one-parameter boolean validator this is roughly adequate: formats, mechanism, and return value are covered. The notable gap is input format expectations, which matter for a validator that will reject malformed strings, and there is no output schema to fall back on.

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 0% for the single 'value' parameter, so the description must compensate, and it largely does not. 'Provide value' adds no meaning: it does not say whether the input must be digits only, whether hyphens/spaces are tolerated, what length is expected per EAN variant, or whether it should be a string vs number.

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 gives a specific verb and resource ('Validate an EAN-8, EAN-13, or EAN-14 barcode number via its check digit') and even names the mechanism (check digit). It stops short of distinguishing this from sibling tools such as x402-ean-validate, x402-ean13-check, or x402-ean, all of which plausibly do the same thing.

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?

There is no statement of when to use this tool versus the several near-identical siblings (x402-ean, x402-ean-validate, x402-ean13-check, x402-upc-check, x402-isbn-validate). The only usage-adjacent text is 'Provide value', which restates the schema rather than guiding selection.

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