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

Annotations already declare the operation as read-only and idempotent. The description adds behavioral context by explaining the three cashback scenarios, monthly breakdown, and clearly stating that the calculation is an estimate with caveats about construction costs and official-site confirmation. This goes beyond the annotations without contradicting them.

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 concise and front-loaded, stating the core purpose in the first sentence, then elaborating on the cashback scenarios. The disclaimer is necessary for an estimation tool but is slightly lengthy; overall it is well-structured without wasted words.

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 explains the return concept (three total amounts with monthly breakdown). It also covers the estimation caveats. However, it doesn't explicitly describe the output format for the monthly breakdown or how parameters like months/mobile_lines influence the calculation, but the schema covers those details. Given the tool's complexity, this is reasonably complete.

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?

All four parameters have descriptions in the schema (100% coverage), so the baseline is 3. The description reinforces the plan_slug source but does not add new semantic details about months, mobile_lines, or mobile_carrier that aren't already in the schema. It doesn't need to compensate, so a 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 simulates the real total cost of a specified hikari plan (光回線プラン) with a monthly breakdown, and explicitly references the required plan_slug from search_hikari_plans. It distinguishes from sibling tools like compare_hikari_plans and search_hikari_plans by focusing on cost simulation for a single plan.

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 indicates the prerequisite of using search_hikari_plans to obtain a plan_slug, giving clear context for when to invoke this tool. It does not explicitly state exclusions or name alternative tools for comparison or switching scenarios, so it falls short of the highest rating.

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