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

Beyond the readOnlyHint annotation, the description discloses important behavioral details: prices are tax-inclusive and discount-free, conditional discounts are returned as separate notes, and the distinction between auto-determined and pre-selected capacity plans. It also warns about rate revisions and directs users to official websites, which is valuable transparency.

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 well-structured into a main explanation and a caveat paragraph. It is somewhat redundant in reiterating the tax-inclusive, no-discount pricing, but each sentence adds clarity. It is appropriately sized for the tool's complexity.

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?

Without an output schema, the description effectively explains the return behavior: plans sorted by price, conditional discounts separated, and the auto vs. pre-selected capacity distinction. It also covers exclusions and the need for verification. This is a complete and self-sufficient description for an agent.

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 100% coverage with descriptions for all three parameters, including examples for monthly_data_gb. The description adds context about how data usage drives plan selection but does not significantly enhance understanding of limit or unlimited beyond the schema. Baseline of 3 is appropriate.

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 searches (検索します) mobile rate plans based on monthly data usage, sorted by price ascending. It specifies the resource type (mobile plans) and distinguishes from sibling tools like search_hikari_plans and search_gas_plans. It also describes the comparison scope and the handling of conditional discounts, making the purpose 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?

The description provides clear context for when to use the tool: when the user needs mobile plan comparisons by data usage. It does not explicitly name alternatives or exclusions, but the resource-specific wording (携帯料金プラン) differentiates it from siblings. No explicit when-not-to-use guidance is given, so a small deduction.

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