validate_vat

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

No annotations are provided, so the description carries the full burden of behavioral disclosure. It explains the two-step behavior: format validation against country-specific patterns and a live VIES lookup, and it lists the return keys including cost_usd. It does not cover failure modes or VIES unavailability, but the core behavior is well disclosed.

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 compact and front-loaded with a one-sentence summary, followed by precise behavioral details and a brief Args/Returns section. Every sentence earns its place and no filler is present.

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 single-parameter tool with no annotations and no output schema, the description covers the input format, validation behavior, and return contract, including the cost_usd field. It could be more explicit about live-service caveats or error behavior, but the essential information needed to call the tool correctly is present.

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?

The input schema provides only a string type with no description, so the description must compensate. It does so by defining the expected format ('VAT number with country prefix') and giving concrete examples such as 'DE123456789' and 'FR12345678901'. This is enough for an agent to format the parameter correctly, though edge cases like case or separators are not covered.

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 'Validate a European VAT number' and gives a specific verb plus resource. It further distinguishes itself from sibling validators by describing the country-specific format check and live VIES database verification, so there is no ambiguity against validate_domain, validate_iban, or other 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 scopes usage to European VAT numbers with country prefixes, which clearly implies when this tool is appropriate. However, it does not explicitly mention alternatives, exclusions, or when not to use it; the guidance is inferred rather than stated.

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