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

Since no annotations are provided, the description carries the entire burden of behavioral disclosure, and it does so impressively. It reveals that the tool may return 'checksum: not-verified' for countries without public algorithms, that it refuses to guess the country, and that it does not query tax authorities. This goes beyond a simple 'validates' claim, offering critical caveats that directly affect output interpretation. The only small gap is the absence of explicit mention of errors for invalid input, but given the coverage, a score of 4 is warranted.

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 dense and content-rich, packing substantial information into two paragraphs. The first sentence front-loads the core purpose and method. The second paragraph provides the rationale and specific edge cases. While it is longer than a terse one-liner, every sentence adds unique and valuable context—no filler or fluff. It could be slightly more structured with bullet points for the list of countries, but this does not detract from overall effectiveness.

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 tool with no output schema and no annotations, the description is quite complete. It explains the validation logic, the varying outcomes, and the critical limitation about not contacting authorities. It also anticipates a common use case (onboarding forms) and explains the design choice to refuse country guessing. The only missing piece is an explicit statement about what the return value looks like or error behavior for invalid countries, which a brief mention would have elevated it further. Nonetheless, it covers the essential aspects comprehensively.

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 input schema already provides 100% description coverage for both parameters, including format and meaning ('ISO 3166-1 alpha-2'). The description adds value by specifying that the 'tin' can include spaces or punctuation, and that 'country' is required to avoid ambiguity. It reinforces the semantic importance of the 'country' parameter by explaining why it is required (the same digits can be valid in multiple places). This exceeds the baseline for high schema coverage, justifying a 4.

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 is exceptionally clear: it explicitly states the tool validates tax identifiers for 25+ countries, specifies the method (checksum where public, format-only where not), and differentiates itself from sibling validation tools like 'eu_vat_validate' and 'tr_vat' by noting it covers multiple country formats and refuses to guess the country. It also provides a concrete list of supported checksum algorithms, leaving no ambiguity about its function.

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 explicitly addresses when to use this tool: for validating tax identifiers from a specific country, especially in scenarios like onboarding forms where garbage data is collected. It also clearly states what it does NOT do (does not ask tax authorities whether the number is issued), which actively prevents misuse. While it doesn't name sibling alternatives like 'eu_vat_validate', the context implies this is a broader, more comprehensive validation, and the specificity of the use case makes the guidance strong.

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