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

Annotations already cover read-only, idempotent, and closed-world behavior, so the description is free to add richer call semantics. It discloses that one code can return multiple towns, explains the town_detail field's parenthesized content, and states that business-specific or PO box codes return offices – all valuable behavioral context beyond the structured metadata.

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 English portion is front-loaded and efficient, covering purpose, input formats, multi-result behavior, and special-case output fields in a few sentences. The bilingual duplication adds length, but given the tool's bilingual title it is purposeful rather than purely redundant.

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 single-parameter lookup with no output schema, the description explains the main return components (prefecture, city, town, town_detail, offices) and the key edge cases. It could be more explicit about the overall response shape or error behavior, but it is largely complete for an agent to invoke correctly.

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 documents the single postal_code parameter with hyphen-optional guidance, so the baseline would be 3. The description adds concrete accepted forms (1000001, 100-0001, 〒100-0001), which extends the schema's meaning with useful input flexibility.

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: looking up a Japanese address from a 7-digit postal code, using Japan Post data. It clearly defines the operation, but it does not differentiate this tool from its sibling search_postal_code, leaving the agent to infer which one to use.

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 implied: use when you have a postal code and need the corresponding address. However, there is no explicit when-not guidance and no mention of the sibling search_postal_code as an alternative, so the agent must infer the boundary from context rather than the description.

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