validate_vat

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

With zero annotations, the description carries full responsibility for behavioral disclosure. It exceeds this bar by detailing return fields (validity, company name/address, confidence level, source), noting that Germany and Spain never expose address data, and explaining input parsing (prefix optional, Greece accepts EL/GR). This gives the agent a clear picture of behavior without needing 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?

The description is two succinct sentences plus an example. It front-loads the purpose, then returns, then input handling. Every clause earns its place; there is no redundant phrasing or filler.

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 the tool's complexity (multiple return fields, edge cases, parameter flexibility) and the absence of annotations, the description covers all necessary ground: what it does, what it returns, how inputs are parsed, and known exceptions. The presence of an output schema reduces the need to detail return structures, but the description does anyway. No critical information is missing.

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 0%, so the description must explain both parameters. It does so explicitly: vat_number can include a prefix or not, and country_code is an optional complement (e.g., 'DE123456789' or '123456789' + country_code='DE'). The Greece EL/GR note further clarifies country_code accepted values. This compensates fully for the lack of 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 opens with 'Validate an EU or UK VAT number via VIES/HMRC', a specific verb and resource that immediately distinguishes it from siblings like validate_iban, validate_eori, and lookup_company_by_vat. The return fields and input flexibility further clarify its unique role among the sibling tools.

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 clearly states the tool's scope (EU or UK VAT) and what it returns, which makes its intended use obvious. However, it does not explicitly name alternatives or provide 'when not to use' exclusions. The lack of direct sibling comparisons is a minor gap, but the purpose is specific enough that an agent can infer when to invoke it.

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