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

Annotations already declare readOnly, idempotent, non-destructive and open-world, so the safety profile is covered. The description adds real value beyond that: it discloses the real-time external dependency, the successful-return payload, and the availability caveat for certain member states.

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: the core behavior and return value come first, and the availability caveat follows. No redundant restatement of the title or schema.

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 no output schema, the description carries the return-value burden and does state the success payload. It omits the invalid-number response shape and the failure behavior when VIES is unavailable, which is a minor but real gap.

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%, so both parameters (vat, country) are already documented, including the country-prefix handling and the EL/XI codes. The description adds nothing about parameter format or interaction, so the baseline 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 (checks/validates) and resource (EU VAT number) and names the exact external authority (European Commission's VIES). It also states the useful outcome (registered company name and address), so an agent can distinguish it from nearby siblings like iban_validate.

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 name and the mention of VIES, but there is no explicit when-to-use statement, no guidance versus alternatives such as iban_validate, and no note about what to do when VIES is down beyond the caveat that it is occasionally unavailable.

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