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

Annotations already cover read-only/idempotent/non-destructive/open-world, yet the description adds substantial behavior beyond them: it runs offline, only 8 countries have a real checksum, checksumChecked distinguishes a verified pass from a format-only pass, and it discloses pricing ($0.002/x402), the prepaid header path, and the free tier. This is exactly the extra context annotations cannot carry.

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?

Front-loaded with the core question, then scope, then payment mechanics; nearly every clause carries signal. It is dense and slightly overlong on the payment/pricing tail, but nothing is truly 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?

With an output schema present, return values need not be described, yet the description still flags the key output field (checksumChecked) and its meaning. Combined with the scope and payment disclosures, an agent has everything needed to call it correctly.

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 100% for the single required parameter, so baseline 3 applies. The description restates the required input (vat) and hints at expected structure (country prefix plus national format) but adds no syntax or normalization detail beyond the schema.

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 (validate) and resource (EU VAT number) plus the exact layers checked: country prefix, national format, and the check-digit algorithm for the 8 named countries. It also sharply delineates what it is not (VIES registration check), so an agent can distinguish it from any registry-lookup sibling.

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?

Explicitly tells the agent when this tool is and is not the right answer ('Registration is deliberately NOT claimed: only VIES can say whether a number is registered'), which is strong scope guidance. It does not name a concrete sibling to redirect to, so it falls short of a 5.

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