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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=false, so the safety profile is covered. The description adds context the annotations do not: the exact return contents (verdict, computed correct check character, country prefix, embedded NSIN) and the pricing model ($0.05 per successful call via x402/USDC on Base), which is materially useful for an agent deciding whether to spend.

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 identifier families and the core action, followed by return contents, scope limitation, and pricing. Every sentence carries information, though the pricing clause slightly interrupts the functional description.

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 no output schema, the description carries the return-value burden and does so explicitly (verdict, correct check character, country prefix, NSIN). All four parameters, both operation modes, and the safety/pricing context are accounted for, leaving nothing an agent needs unstated.

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, nsin, value, and country fully. The description restates the build path ('a country code plus a 9-character NSIN') but adds no format or edge-case detail beyond the schema, so the baseline 3 applies.

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?

Names specific verbs (validate check digits, build an ISIN) and concrete resources (ISIN per ISO 6166, CUSIP, SEDOL, FIGI). This clearly distinguishes it from sibling validators like gtin_barcode_validate, iban_validate, and vat_tax_id_validate, which cover different identifier families.

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?

Explicitly bounds the tool's scope with 'Format and checksum only, no security master lookup,' which tells the agent when this is and is not sufficient. It does not name a specific alternative tool for the lookup case, so it stops short of full when/when-not routing.

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