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

Beyond the readOnlyHint annotation, the description explains return semantics: whether the number exists and, where published, the name behind it. It also discloses a critical failure mode: VIES downtime is reported as disponible=false, which is not the same as an invalid number.

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 front-loaded with the primary lookup action, then adds only the high-value caveats about tax conclusions and VIES availability. Every sentence earns its place and there is no repetition.

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 read-only lookup with no output schema, the description supplies the missing return semantics and the important distinction between unavailable and invalid. Together with the annotations and fully documented schema, an agent has enough information to call and interpret this tool 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 both the country code and number parameters, including that the country can be omitted when the number carries its prefix. The description does not add parameter-level syntax or format guidance beyond what the schema provides.

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: looking up an EU VAT number in the European Commission's VIES registry. It distinguishes the scope from Spanish tax ID or VAT calculation tools by naming VIES and EU VAT explicitly.

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 gives clear context and an important limitation: the result does not determine whether an invoice may be issued without IVA, which is the user's decision with their gestoría. It does not explicitly name sibling alternatives such as validate_spanish_tax_id or calculate_vat, so it falls short of full alternative routing.

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