check_vat

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It explains the return behavior (valid/invalid, name/address if available), the combined vs. separate input forms, and the special consultation number mechanism for Polish JDG. It also warns about when VIES will not issue a consultation number, which is a critical behavioral nuance.

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 moderately long but each sentence contributes information. It front-loads the core purpose and return value, then details the input forms and the special case. There is no filler or repetition, though it could be slightly trimmed without losing critical detail.

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 (5 parameters, two input modes, optional requester fields, a special legal-proof mechanism), the description covers all necessary aspects: input variants, return format, and the condition for obtaining a consultation number. Without an output schema, the description adequately explains what the agent can expect from the call.

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 schema description coverage is 100%, so the baseline is 3. The description adds meaningful context beyond the schema: it explains the relationship between requester_country and requester_vat, emphasizes the combined 'vat' parameter's precedence, and provides the Greek/Northern Ireland country code exceptions. This exceeds the baseline by adding practical usage semantics.

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's function: checking an EU VAT number against the VIES database. It specifies the verb 'Check', the resource (EU VAT number), and the validation scope (VIES). It also distinguishes itself from the sibling tool (list_supported_countries) by focusing on validation rather than listing countries.

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 provides clear context on when to use the tool (to check VAT numbers) and even includes a specific use case (Polish JDG issuing 0% VAT invoices). However, it does not explicitly discuss alternatives or exclusions relative to list_supported_countries, though the purpose difference is implicit. Missing a direct 'when not to use' guidance, but the context is sufficient.

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