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

No annotations exist, so the description carries full burden. It discloses real-time query via VIES, 24h caching, pricing ($0.005 per call), and returned fields (is_valid, name, address, request_date). This provides good behavioral context beyond the schema, though some details like error handling are omitted.

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?

The description is 4 sentences, front-loading the core purpose and adding necessary details. It is efficient but includes pricing information that, while useful, slightly lengthens it. No superfluous content.

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?

Despite no output schema, the description explains return values (is_valid, name, address, request_date), caching behavior, pricing, and API source. For a simple validation tool with two parameters, this is comprehensive and leaves minimal 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?

Schema has 0% coverage with no parameter descriptions. The description adds critical meaning: country must be member state code (including EL/XI), VAT number without country prefix, and provides a concrete example. This fully compensates for the missing schema descriptions.

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 clearly states the verb 'validate' and the resource 'intra-EU VAT number' via the VIES API. It distinguishes from sibling tools like validate_email or validate_iban by specifying intra-EU VAT, making its purpose unambiguous.

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 implies usage for EU VAT validation but does not explicitly state when to use it over other validation tools (e.g., other country-specific tools). No exclusion criteria or alternative recommendations are provided, leaving usage context somewhat inferred.

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