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

With no annotations, the description carries the full burden and does well: it discloses per-lookup pricing (€0.004), that billing goes against the Bearer API key, and that name/address are only returned when the member state supplies them. It does not cover failure behavior (malformed vs. unregistered number) or rate limiting, which keeps it below a 5.

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 capability, cost, and auth model are front-loaded in the first sentences, with parameter notes trailing in a compact clause. Dense but free of filler; the only mild cost is that the parameter guidance is packed into one long dash-introduced sentence rather than being separately scannable.

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 two-optional-parameter lookup with an output schema present, the description covers what the tool does, its coverage limits, its cost model, and the shape of the returned enrichment, which is enough to call it correctly. The remaining gap is error/edge-case behavior, a minor omission given the output schema carries the return structure.

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% and both parameters are nullable with no required fields, so the description must compensate. It does clarify that vat accepts an optional country prefix and that country is the ISO-2 fallback, but it never states the precedence when the prefix and country disagree, nor whether both may be null.

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 resource and operation: validity of an EU VAT number via VIES, scoped to 27 EU states plus XI. The inclusion of 'registered company name and address' further distinguishes it from siblings like iban_validate or company_verify, so an agent can identify it without opening the schema.

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?

Usage is implied by the VIES/EU VAT framing, and the cost line gives a reason to avoid redundant calls, but there is no explicit when-to-use/when-not-to-use guidance or named alternative (e.g. company_verify) for non-EU or non-VAT checks. The agent must infer the boundary conditions.

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