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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, covering the safety profile. The description adds context that it is an 'Official European Commission' API, implying authoritative and reliable, but does not disclose additional behavioral details such as rate limits, error handling, or response specifics. With annotations this strong, a 3 is appropriate as the description adds minimal extra value beyond the annotations.

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 description is a single, concise sentence: 'VAT validation API / VIES API. Official European Commission EU VAT check.' It is front-loaded with the core purpose and contains zero filler. Every word earns its place.

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?

Given the tool's simplicity (2 well-documented parameters), the presence of an output schema, and strong annotations, the description is complete enough. It identifies the authoritative source (European Commission/VIES) and the purpose. The output schema covers return values, and annotations cover safety, so no further contextual information is necessary.

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?

The input schema already provides full descriptions for both parameters (vat_number, country_code) with schema coverage at 100%. The description does not add any additional meaning beyond what the schema already states, so it meets the baseline of 3 but does not exceed it.

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?

The description clearly states it is a VAT validation API, specifically the VIES API from the European Commission. It specifies the resource (EU VAT check) and the action (validation), effectively distinguishing it from sibling tools that deal with AI act applicability, compliance preflight, OFAC screening, and legal entity verification, which are unrelated to VAT validation.

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 implies usage for EU VAT validation ('Official European Commission EU VAT check'), but does not explicitly state when to use it vs alternatives or provide exclusions. Since the sibling tools are unrelated to VAT, the need for explicit alternatives is low, but the description still lacks explicit guidance on when to invoke this tool.

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