Validate EU VAT number

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

Discloses that it connects to VIES, returns metadata, and optionally provides a consultation number for audit trails. No destructive behavior; API key requirement and country limitations are stated.

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?

Concise paragraph with clear flow: main purpose, special case, when to use, prerequisites and limitations. No redundant sentences.

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?

Fully describes output (validity, company info, registration date, consultation number) and constraints (EU-27+XI, API key). No output schema, so description carries complete burden.

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?

Adds meaning beyond schema: explains requesterCountryCode/requesterVatNumber pair yields consultation number, and that vatId has spaces/dashes stripped. Schema coverage is 100% but description enriches it.

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 tool verifies EU VAT numbers against VIES, listing specific returned data. It is distinct from siblings like check_vat_format (format only) and others (rates/lists).

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 states use cases: confirm VAT real, look up company, evidence for compliance. Also mentions API key requirement and supported countries, providing clear context for when to use.

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