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

With no annotations, the description carries the disclosure burden and does well on outputs (valid, name, address, requestDate) and cost ($0.01 USDC). It says nothing about failure behavior — what comes back for a malformed or unregistered VAT number, whether the call is billed on failure, or how payment is authorized.

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 compact sentences: purpose and source first, input format second, pricing and cross-sell last. The pricing/upsell clause is arguably promotional but does carry decision-relevant information, so little is wasted.

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?

Adequate for a simple single-purpose lookup: return fields are enumerated in lieu of an output schema and the input shape is illustrated. However, an agent still lacks error/failure semantics and the parameter-requiredness picture for a 3-parameter, zero-required tool with no annotations.

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 coverage is 0%, so the description must compensate. It explains the `vat` format with concrete examples and the EL-for-Greece quirk, and hints at a country+number alternative, but leaves the separate `number`/`country` pair underspecified and never says that no parameter is actually required or which combinations are accepted.

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 and resource (validate an EU VAT number), names the authoritative source (official VIES), and enumerates the returned fields. It also contrasts itself with the broader /kyb sibling, so an agent can tell which one to reach for.

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?

"Use this to validate an EU VAT number" establishes the context, and the closing "Related: Verify the whole business: /kyb" routes an agent to the wider option when only a VAT check won't suffice. It stops short of an explicit when-not (e.g. non-EU or non-VAT registry lookups covered by company_full/uk_company).

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