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

Annotations already declare readOnlyHint=true and idempotentHint=true, indicating safe, read-only operation. Description adds crucial context that validation is offline and does not confirm registry existence, enhancing transparency beyond 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?

Two concise sentences plus a bolded caveat. Every sentence is purposeful—states function, format, limit, and alternative. Front-loaded with key info, no waste.

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 validation tool with 1 parameter and output schema (implied), description covers what the tool does, its limitation, and when to use sibling. No gaps remain.

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 has 100% coverage with description of parameter 'number' (example provided). Description adds value by specifying format 'T + 13 digits' and mentioning check-digit validation, which is not in schema description. Baseline is 3, but the added context justifies 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?

Description clearly states it validates Japanese qualified invoice number format and check digit, specifying 'T + 13 digits'. It distinguishes from sibling tool invoice.verify by noting it does NOT confirm existence in NTA registry, so purpose is highly specific and unambiguous.

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 states when to use (offline format/check-digit validation) and when not to use (for real registration, use invoice.verify). Provides clear alternative, enabling correct tool selection.

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