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

With no annotations, the description carries the full burden and adds meaningful behavioral context: it says the data is verified from official pages, limited to tax-included regular prices (excluding discounts), and refreshed weekly. These are real caveats beyond what the name or schema express, helping an agent judge freshness and scope before calling.

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 description is a single compact Japanese sentence that front-loads the topic, then packs the essential constraints and field list without wasted words. Every clause earns its place.

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 parameterless, read-only data lookup tool, the description is largely complete: it states the subject, geographical scope, source reliability, price type, update frequency, and available fields. It lacks only explicit routing guidance to sibling tools, but that is already covered under usage guidelines.

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 has zero parameters, so the baseline is 4. The description adds value by listing the data fields an agent can expect (company, plan, GB, monthly yen), which partially compensates for the absence of an output schema, even though there are no parameters to document.

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 identifies the resource: Japanese mobile phone rate data, with explicit scope ('official pages', 'tax-included regular rates only') and expected fields (company, plan, GB, monthly yen). It is more specific than the tool name alone and implicitly distinguishes itself from sibling tools like hikaku or cheapest by emphasizing 'regular rates only', though it does not explicitly name any alternative.

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?

There is no guidance on when to use this tool versus siblings such as hikaku, cheapest, or chingin. The description states data constraints (official, tax-included, weekly-checked) but does not explain when an agent should select mobile_plans over alternatives or when it should not.

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