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

Annotations already cover safety (readOnly, idempotent, non-destructive, open-world), so the bar is lower, yet the description still adds value by disclosing the return shape: validity plus trade name and address 'where the member state exposes it'. That caveat about partial data coverage is genuinely useful and not derivable from the schema.

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 compact sentences plus an example: purpose front-loaded, return behavior second, no filler. The example duplicates the schema's own example, a minor redundancy that keeps it out of the top band.

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?

With one fully documented parameter and no output schema, the description carries the return-value explanation and the important caveat that name/address are not always returned. It omits error behavior (malformed vs. unregistered number) and any rate-limit or availability note for this external service, which would have made it complete.

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 parameter is fully documented, so the baseline applies. The description repeats the same example (BE0403170701) already present in the schema and adds no new syntax or format guidance.

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 (validate) and resource (EU VAT number) against a named authority (the official EU VIES service), which an agent can distinguish from sibling validation tools like iban_validate or kbo_lookup. No sibling is named explicitly, so it falls just short of the top score.

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 context of use is clearly implied by 'validate an EU VAT number', but the description never states when to prefer this over iban_validate/kbo_lookup or what preconditions apply (e.g. country prefix required, VIES availability). Usage is inferable, not prescribed.

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