Validate an EU VAT number via VIES (paid $0.003)

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

With no annotations, the description fully discloses the cost ($0.003, USDC on Base), the ~1h caching to handle flaky VIES, the return of company name/address, and the 402 challenge when unpaid. This is exceptional transparency.

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 dense and front-loads key information (cost, purpose). Minor promotional language ('FLAGSHIP') and some redundant phrasing keep it from being perfectly concise, but every sentence contributes.

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?

The description covers the main action, payment flow, and expected outputs (valid/registered, company info, or challenge). Although there is no output schema, the description adequately sets expectations for return values and behavior.

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?

The input schema already documents all three parameters with 100% coverage. The description adds minimal semantic value beyond restating the required inputs and the role of x_payment, so it meets the baseline without exceeding it.

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 clearly states the tool validates EU/EEA VAT numbers with a specific verb and resource. It details the two-stage process (format/checksum pre-check plus live VIES lookup) and implicitly distinguishes it from sibling validators like validate_iban and validate_email.

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 on how to use the tool, including the payment prerequisite and the fallback to a 402 challenge. It does not explicitly name alternatives, but the domain-specificity makes the intended use obvious.

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