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 full burden, and it does disclose the shape of what comes back (country code plus three naming variants). It does not say whether the list is static, cached, or sourced live from VIES, which is the one behavioral fact that would matter for a reference-data tool.

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?

Two short sentences with the resource front-loaded and concrete examples (USt-IdNr., Partita IVA, btw-nummer) that make the output tangible. The English sentence is largely a restatement of the Chinese one, which is mild redundancy but appropriate for a bilingual audience.

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 no output schema, annotations, or parameters, the description is the sole carrier of information, and it does describe the returned fields. Nothing an agent needs in order to invoke this trivial zero-arg tool is missing, though origin/freshness of the list is unstated.

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 tool takes zero parameters, so the baseline of 4 applies. There is nothing for the description to clarify beyond the empty 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?

The description states a specific verb and resource ('return the list of countries that support VAT validation') and enumerates the returned fields (code, native name, local VAT term, Chinese/English names). It does not explicitly distinguish itself from siblings like validate_vat, but the noun phrase 'list of countries' makes the scope unmistakable.

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?

Usage is only implied: an agent would infer this is a reference lookup to run before or alongside validate_vat/check_vat_format. There is no explicit when-to-use statement, no mention of the sibling tools, and no guidance on whether this list should gate validation calls.

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