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

With annotations already declaring readOnly/idempotent/closed-world, the description still adds material behavioral context: it discloses the return payload (verdict, computed correct character, country prefix, NSIN) and the transaction model ($0.05 per successful call, paid over x402 in USDC on Base). Cost and payment mechanism are behaviors an agent cannot infer from 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?

Front-loads the core purpose, then the return values, then the build branch, then scope limits and price. Every sentence carries distinct information with no redundancy.

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?

No output schema exists, so the description must carry return semantics – and it does, enumerating verdict, corrected check character, country prefix and NSIN. Combined with the cost disclosure and scope limit, an agent has everything needed to invoke it correctly.

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 100%, so the schema already documents op, value, country and nsin. The description does add the semantic relationship of the build-isin branch (country code + 9-character NSIN), but per the baseline rule with full coverage this remains a 3.

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?

States specific verbs (validate, build) against named identifier standards (ISIN ISO 6166, CUSIP, SEDOL, FIGI), so the resource is unambiguous. An agent can immediately tell this apart from sibling identifier tools like id_gs1 and id_national.

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?

"Format and checksum only, no security master lookup" clearly delimits the scope of use, telling the agent this is a syntactic validation, not a data retrieval. It does not explicitly name alternative sibling tools, but the domain boundary is clear enough to route correctly.

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