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

Annotations already indicate readOnlyHint, idempotentHint, and non-destructive behavior. The description adds value by stating it's real-time via VIES and detailing the return fields: {valid, name, address, vat_number, country}. No contradictions 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?

The description is a single, front-loaded sentence with no filler. Every word contributes: nature (real-time), source (VIES), scope (27 countries), and return structure. It is optimally concise for the agent.

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 simple lookup tool with one parameter family, the description covers the core functionality and output fields. The presence of an output schema further complements completeness. No gaps are evident given the tool's complexity.

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%; each parameter already has a description. The tool description does not add extra meaning beyond the schema, but the brief note on return format indirectly hints at how the parameter affects results. Baseline 3 is appropriate given full schema coverage.

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 performs 'Real-time EU VAT validation via VIES (27 countries)', specifying the verb (validate) and resource (EU VAT numbers). It distinctly separates this from sibling tools like entity_lookup or get_competitors, which serve different purposes.

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 implies usage when needing to validate an EU VAT number, but provides no explicit guidance on when not to use it, prerequisites, or alternatives among siblings. All parameters are optional in schema, yet logically one must be provided—the description does not clarify this constraint.

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