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

Annotations already cover read-only, idempotent, non-destructive behavior, so the description only needs to add context. It adds the data source, MIT license, and no-auth requirement, plus the deterministic 'all 47 prefectures' scope. No contradiction with 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?

Three tight sentences: what the tool returns, how to use its output downstream, and source/license/auth context. No wasted words, and the most important purpose is front-loaded.

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?

Given a simple zero-parameter reference tool with an output schema and safety annotations, the description covers content, downstream usage, source, and access requirements. Nothing an agent needs to select and invoke it correctly is missing.

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 tool has no real parameters, and the schema placeholder already states 'No parameters required.' The description doesn't add parameter-specific detail, but none is needed for a zero-required-parameter tool, so the baseline score of 4 applies.

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 clearly states the tool returns the complete list of all 47 Japanese prefectures, including JIS codes, names in kanji, hiragana, katakana, English, and geographic region. This specific resource and content set distinguishes it from generic country-list tools and other postcode-japan siblings.

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?

It explicitly tells agents to use the returned prefecture codes as the 'prefecture' filter in postcode-japan.postal.search, giving a concrete integration workflow. It does not mention when not to use the tool or alternative lookup paths, so it falls just short of fully explicit when-not guidance.

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