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

Annotations already cover read-only/idempotent/no-open-world, so the bar is lower; the description adds real behavioral context by warning that one code can map to several towns and that business/PO-box codes surface in `offices`. It does not say what happens for an unknown or malformed code, which is the remaining gap.

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?

Front-loaded with the action and input format, and each sentence carries information. The fully duplicated bilingual text doubles the length, which is a convention rather than waste, but it is not maximally tight.

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, the description usefully explains the key return fields (`town_detail`, `offices`) and the possibility of multiple results, so an agent knows what to expect. It stops short of describing error handling or result ordering.

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?

With a single parameter and 100% schema coverage the baseline is 3, but the description adds accepted input variants (bare 7 digits, hyphenated, and the 〒 prefix) that the schema's 'hyphen optional' does not enumerate.

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?

States a specific verb (look up) and resource (address for a 7-digit Japanese postal code) with the exact data source (Japan Post). The exact-code input requirement implicitly separates it from the sibling search_postal_code, which would take free-text address input.

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 required input form ('accepts forms like 1000001, 100-0001 or 〒100-0001') implies when the tool applies, but the description never explicitly names search_postal_code as the alternative for the reverse or fuzzy lookup. Usage is inferable but not stated.

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