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

Annotations already declare readOnly/idempotent/openWorld, so the bar is lower; the description still adds substantive behavior: it discloses the partial-return rule (registered name only when the member state shares it) and the critical failure mode — a busy or down member state surfaces as an error, never as an invalid number. That error-vs-invalid distinction is exactly the kind of nuance an agent needs and cannot infer.

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: what it does, what it covers geographically, and the error-vs-invalid caveat last. Every sentence earns its place and the important scope constraint 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?

With no output schema, the description still explains the return shape (live/invalid plus optional registered name) and the error case. For a single-parameter lookup tool, nothing an agent needs to call it correctly or interpret results is missing.

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 already documents the two-letter country-code prefix, ignored whitespace/dots/hyphens, and the GR→EL mapping. The description only reinforces that it takes 'one' number (no batch), adding marginal meaning beyond the schema, so the 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 (checks/validates), a specific resource (one VAT number) and the authoritative source (European Commission VIES). No sibling tool shares this domain, and the description makes the scope unambiguous without opening the 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?

The description sets clear applicability boundaries — the 27 EU states plus Northern Ireland (XI) — which tells the agent when the tool is and isn't valid (e.g. not for US EINs). It doesn't name alternatives, but no sibling overlaps this task, so there is little to route between.

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