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

With no annotations, the description carries the full burden and does well: it discloses that this is a light read, its cost (0.2 credit), the upstream source, and that name/address are returned only when the member state supplies them. It stops short of failure modes (VIES downtime, malformed input) or rate-limit behavior.

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?

Three tight sentences: purpose/source first, then input format, then cost. Every sentence earns its place and the most important information (what it validates and what it returns) is front-loaded.

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?

For a one-parameter validator with no output schema, the description covers the essential return semantics (valid flag plus name/address when available) and the input format. Only ancillary details like error/indeterminate results are unaddressed.

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 with the same example ('FR40303265045') repeated in the description. The description adds no formatting or edge-case detail beyond what the schema already provides, so 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 (validates), resource (EU VAT number), and the authoritative external source (VIES / European Commission). It is clearly distinguishable from siblings like validate_iban and verify_company without opening any schema.

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 purpose — validate a VAT number when you need its current validity — and the cost note hints it is a cheap check. However, there is no explicit when-to-use/when-not guidance and no routing to or from alternatives such as validate_iban or verify_company.

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