Lookup Tax ID via VIES (EU VAT Validation)

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

Annotations (readOnlyHint, destructiveHint, idempotentHint) are already clear. Description adds that it queries the official EU registry and returns specific fields. Could mention network dependency but adequate.

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?

Description is short, bilingual, and includes example. Every sentence is useful. No wasted words.

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?

With no output schema, description explains return values (company name, address, validity status). Usage context (EU invoicing compliance) is provided. Complete for a lookup tool.

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 coverage is 100%, but description adds value by specifying vatNumber must be without country prefix and countryCode is ISO 3166-1 alpha-2. Example further clarifies usage.

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?

Description clearly states the tool validates EU VAT numbers via VIES, returns company name, address, and validity status. Verb 'Lookup' and specific resource 'VAT ID' are well-defined. No sibling tool has similar purpose, so no confusion.

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 says 'Essential for intra-EU invoicing compliance — always validate before adding a EU client.' Provides an example. Absence of alternative tools is acceptable as no sibling offers similar functionality.

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