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

Annotations already indicate read-only, idempotent, open-world behavior, but the description adds substantial operational context: it is offline and free, catches single mistyped characters that format checks miss, and reports country-of-issuance as an observation rather than a fraud verdict. These caveats meaningfully shape agent expectations beyond the annotations.

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, each earning its place: the use case and scope, the core verification capability, and the critical limitations. The most decision-relevant information is front-loaded, and there is no fluff or repetition of schema details.

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 simple two-parameter verification tool with rich annotations, the description covers inputs, scope, behavior, limitations, and optional country logic. Even without an output schema, an agent has enough context to invoke it correctly and interpret the result's intended meaning.

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. The description adds value by explaining that expectedCountry controls whether the response reports country-of-issuance and clarifying that this is an observation, not a fraud verdict. The iban parameter is already well documented in the schema, so the description's contribution is helpful but not essential.

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 states a specific verb and resource: 'Check a payment account number (IBAN)' before paying or sending an invoice. It names the exact standards verified (ISO 13616 structure, ISO 7064 check digits) and covers 70+ countries, which cleanly distinguishes it from sibling tools like check_vat_list even without naming them.

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 explicitly says when to use the tool ('before paying or before sending an invoice') and what it does not do ('It never claims the account exists or who owns it'). This gives an agent clear selection and exclusion criteria, even though it does not name sibling tools.

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