lookup_company_by_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. It only states the action (lookup) without disclosing whether it is read-only, what happens on invalid or unmatched VAT numbers, or any rate-limit or authentication requirements. For a lookup tool, this omission is noticeable, though not as severe as for a mutation tool.

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?

Two sentences with no redundant words. The main purpose is front-loaded in the first sentence, and the second provides essential usage context. Every phrase earns its place.

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?

An output schema exists, so return-value details need not be in the description. The description covers the core use case and fallback scenario, but it omits potential edge cases like non-German/Spanish VAT numbers or error behavior. Given the single parameter and the presence of an output schema, this is adequate but not thorough.

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 0% for the single parameter 'vat_number'. The description mentions 'by VAT number' but does not specify the expected format (e.g., country code prefix, digits only, length). It adds only minimal meaning over the schema's type string and title, leaving the agent to guess the exact input format.

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 gives a specific verb and resource ('Look up company data by VAT number') and adds the GLEIF cross-reference to clarify the source. It clearly distinguishes itself from siblings like validate_vat and lookup_company_by_lei by focusing on company data retrieval rather than validation or LEI-based lookup.

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 states a clear condition for use: as a fallback for German/Spanish VAT numbers when VIES returns no company name/address by law. This tells the agent when to use it, though it doesn't explicitly name alternative tools to prefer (e.g., validate_vat for validation). The context is clear but not as explicit as a direct sibling-routing statement.

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