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

Annotations already declare readOnly, non-destructive, open-world behavior, lowering the burden. The description still adds real context beyond them: it discloses the returned fields, explains that the consultation number serves as audit evidence, and flags the behavioral exception that Norwegian numbers are checked against the Norwegian VAT register rather than VIES.

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?

Front-loads the core action and the authoritative source, then folds outputs and the Norwegian exception into a single dense sentence before the use-when clause. Well structured, though the enumeration of return fields is slightly list-heavy.

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 cover the essentials: validity verdict, name and address, name-match result, and consultation number. It does not mention error conditions (invalid format, service unavailable) or rate limits on the external VIES call, which would round it out.

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 all three parameters are already documented in the schema, including the country-prefix format and the not_checked/match/partial/mismatch outcomes. The description restates the optional name check and requester-VAT consultation number but adds no syntax or format detail 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 (validate) and resource (EU VAT number) against a named authority (VIES), and enumerates what comes back: valid/invalid status, registered name and address, name-match check, and consultation number. An agent can immediately tell this apart from siblings like company, company-de, sanctions, and verify.

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 gives explicit trigger scenarios: 'Use when: onboarding an EU supplier or customer, or before paying an invoice.' That is clear context for selection, but it names no alternatives or when-not conditions (e.g. non-EU entities, or when full company data is needed versus a pure VAT check).

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