list_supported_countries

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

No annotations are provided, so the description carries the burden. It clearly describes a read-only listing operation with no side effects, and specifies the exact set of codes. It doesn't state the return format, but for a zero-parameter list tool the implication of returning an array of strings is strong enough. Slight gap in not explicitly stating it is safe/read-only, but it is entirely inferred from the verb 'List'.

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?

A single sentence front-loads the action, resource, and scope without any wasted words. Every element ('List', 'country codes', 'VIES', the list of codes) earns its place. No redundancy or filler.

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 zero-parameter, no-output-schema tool, this description is complete. It tells the agent exactly what the tool returns (the set of supported codes) and the extent of that set. There is nothing an agent needs to know to call it correctly that is missing.

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?

There are zero parameters, so the baseline per rubric is 4. The description adds no parameter information because none is needed; the tool takes no input and simply returns a static list.

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 ('List'), a specific resource ('country codes supported by VIES'), and the exact scope (27 EU member states plus XI). This unambiguously distinguishes it from the sibling check_vat, which performs a different operation.

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?

Although it doesn't explicitly name check_vat, the purpose is self-evident: use this tool when you need the set of supported country codes. There is no alternative for listing countries, so no exclusion is needed. The context makes the appropriate usage clear without ambiguity.

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