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

Goes well beyond the readOnlyHint/openWorldHint annotations: discloses billing behavior (1 document per valid/invalid, unavailable uncharged), retry policy, cache avoidance, and a security directive to treat provider messages as untrusted data. No contradiction with annotations.

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?

Every sentence carries distinct operational value: evidence, outcomes, retry/cache, provider, document charges, API key, and security guidance. It is front-loaded with the core purpose and remains dense without padding.

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?

Covers invocation requirements, billing, outcome classes, and the untrusted-data caveat. There is no output schema, and the description does not spell out the exact response shape, but the dated-evidence concept and outcome taxonomy give an agent enough to call and interpret the result 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?

The schema already fully describes the sole required parameter, including the GR-to-EL normalization, so the description does not need to add much. It confirms the parameter is the VAT number sent to VIES, but adds no new semantic 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 and resource: live VAT-number verification against the European Commission VIES service. It distinguishes itself by naming the exact outcome categories (valid, invalid, unavailable) and by making clear this is not a tax determination.

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?

Clear context: live verification, one bounded retry, no Attestwire cache, and API key requirement. It includes an explicit exclusion ('Not a tax determination') but does not name sibling alternatives such as check_vies_status for service availability.

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