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

With no annotations, the description carries the full burden. It discloses that the tool performs live real-time queries, has a 24h cache, costs $0.005 USDC per call, and returns specific fields (is_valid, trader name/address, request_date). It also notes conditional behavior (trader info only when member state publishes it). This is strong behavioral disclosure despite missing rate limits or auth details.

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 two sentences but information-dense, front-loading the core purpose then systematically covering query format, return values, caching, and pricing. No filler or redundancy—each clause adds essential context.

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?

Despite lacking an output schema and annotations, the description explains the return fields, the API mechanics, and the pricing. It does not cover edge cases like invalid country codes or network errors, but for a validation tool this is adequate. It's nearly complete, though it could mention what happens if the VAT number is invalid or the registry is unavailable.

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 0%, so the description must compensate. It fully explains both parameters: cc as member state code (including EL/XI) and vat as the number without country prefix, illustrated with the example ?cc=IE&vat=6388047V. This adds significant meaning beyond the bare 'country' and 'vat' property names in the schema.

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 states a specific verb+resource: 'Live intra-EU VAT number validation via the European Commission VIES REST API'. It clearly distinguishes itself from sibling tools like validate_email and validate_iban by focusing on EU VAT numbers, and even specifies the API and registry used.

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 provides clear context by showing a concrete query example and explaining parameter format (cc member state code, vat without prefix). It implicitly indicates when to use the tool (when an EU VAT number needs validation) but does not explicitly mention alternatives or exclusion scenarios. This is a minor gap.

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