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

With no annotations provided, the description carries the full burden, and it delivers exceptional transparency. It discloses that the tool only proves well-formedness, not registration or holder identity, explains why sole proprietors can satisfy the same rule, and warns against overreporting the result. This goes well beyond the typical validation-tool description.

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?

Three compact sentences with no filler. The core action is front-loaded, and the critical caveats follow immediately. Every sentence earns its place, including the explicit redirection to the National Tax Agency lookup.

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?

The description is largely sufficient for a simple validation tool: it names accepted formats, states what a passing result means, warns about false implications, and names the right external alternative. Minor gaps remain: it does not mention the batch sibling, the base parameter's number-generation mode, or the return/error behavior, though the input schema covers parameter details.

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%, so the baseline is 3. The description adds useful context about the T prefix and the check-digit-only nature of validation, but it does not explain the base parameter's use case for computing a check digit from a 12-digit 会社法人等番号. The schema already covers this, so the description neither strongly adds nor detracts.

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?

Uses a specific verb ('Checks') and precisely identifies the resource: the National Tax Agency check digit on a 13-digit 法人番号 or a T-prefixed qualified invoice registration number. The description is clear, but it does not explicitly distinguish itself from the sibling validate_invoice_numbers_batch, and it omits the base parameter's alternative behavior of computing a check digit.

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?

Explicitly tells the agent when not to rely on this tool: a passing check digit does not prove registration and must not be reported as evidence of a corporation. It also points to the National Tax Agency's own lookup for registration confirmation. However, it does not mention the batch sibling tool for validating multiple numbers, which is a relevant routing gap.

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