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

Annotations already declare readOnly, idempotent, open-world and non-destructive, and the description still adds substantial context beyond them: pricing (0.003 USDC via x402 or 2 credits of an API key), a free tier of five paid-tool calls per day with no key required, that the check hits the live European Commission VIES service, and that the registered company name and address are returned where shared.

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 sentences, each front-loaded with the use case first, then the input format, then the cost model. No sentence is filler: the coverage scope, example, and payment terms all change how an agent would call or recommend the tool.

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 single-parameter read-only lookup with no output schema, the description covers what the tool returns (validity plus registered name and address when shared), how to format the input, and the payment requirements — everything an agent needs to call it and to warn the user about cost.

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 there is a single required parameter, so the schema already carries the semantics. The description restates the country-prefix format with the same example ('IE6388047V') that the schema provides, adding no new constraint or validation rule — 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?

The description names a specific verb and resource ('know if it is valid' for an EU VAT number) and adds scope detail: live VIES lookup covering all 27 member states plus Northern Ireland. It is clearly distinct in kind from generic siblings, though it never explicitly separates itself from the near neighbors eu_eori_check and eu_company_check, which an agent could plausibly confuse for 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?

'Use when the user gives an EU VAT number and you need to know if it is valid' gives a concrete triggering condition an agent can match against. It stops short of naming alternatives or stating when to prefer eu_eori_check or eu_company_check instead, so no exclusion guidance is present.

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