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

With no annotations provided, the description carries the full burden and excels: it discloses pure offline computation, 1y cache, pricing ($0.001 USDC), return fields (valid, country, bban, failure reason), and tolerance for spaces/dashes. This fully informs the agent of behavior without requiring 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?

Every sentence adds value: purpose, algorithm, example, tolerance, return values, offline/cache, and price. It is dense yet efficient, with no repetition or filler, making it easy to scan and digest.

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?

Given no output schema and no annotations, the description comprehensively covers purpose, usage context, input format, output fields, offline behavior, caching, and pricing. This is sufficient for an agent to select and invoke the tool correctly without ambiguity.

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?

The input schema provides only a string parameter with 0% coverage, but the description compensates with a concrete example (?iban=FR1420041010050500013M02606) and explicitly notes spaces/dashes are tolerated. This gives the agent unambiguous guidance for constructing a valid request.

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 explicitly states 'Deterministic IBAN check' and details the checks (format regex, per-country length, ISO 7064 mod-97 checksum). This clearly identifies the tool's function and distinguishes it from sibling validators like validate_email, validate_vat_eu, and validate_siret_fr by name and algorithmic specifics.

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?

The description provides a clear use case: 'before a SEPA/international transfer.' This contextual signal tells the agent when to invoke the tool. However, it doesn't explicitly mention alternatives or when not to use it, which is a minor omission given the tool's obvious domain-specificity.

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