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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety and network behaviour are covered. The description adds genuine context by disclosing what comes back (validity, registered name, address) and that supplementary fields appear only 'where the member state publishes them', warning the agent that results vary by country.

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 tight sentences, front-loaded with the core action and followed by return semantics. Every clause carries information; nothing is redundant with the schema or annotations.

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 read-only lookup with no output schema, the description covers purpose and return values adequately, including the important caveat about member-state-dependent fields. It stops short of covering rate limits, error behaviour for malformed prefixes, or expected latency.

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 description coverage is 100% and the single 'vat' parameter is fully documented there, including country-prefix format, space/dot handling, and the EL/XI special cases. The description adds no parameter-level meaning beyond the schema, so the baseline of 3 applies.

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?

States a specific verb (check/validate) and resource (EU VAT number), and adds the scope qualifier 'valid and active'. It is easily distinguished from siblings like iban_validate or lei_lookup, which validate different identifier types.

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 purpose implies when to use it (VAT number verification), but the description names no alternatives, prerequisites, or when-not-to-use conditions relative to sibling validators such as iban_validate or nordic_company_lookup.

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