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

Annotations already declare readOnlyHint/idempotentHint/non-destructive, so the bar is lower; the description nonetheless adds real value by specifying the currency and tax treatment (日本円・税込), the eligibility-verification behavior, and concrete unsupported prefixes (携帯電話番号・0990・0570). It does not describe failure messages or rate limits.

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, front-loaded with what is computed, then the eligibility caveat, then the safety reassurance. Every sentence carries distinct information and nothing is padding.

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 does the work of explaining the return content: a JPY tax-included price plus destination-compatibility. That covers the essentials for a 2-param lookup, though it could say how an unsupported number is surfaced (error vs. flag).

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?

Schema coverage is 100%, so both fax_number and pages are already fully documented, including format examples and the 1–100 range. The description only restates that price derives from destination and page count, adding no syntax or constraint detail beyond the schema — the baseline 3.

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 (confirm/check) and resource (FAX price in JPY, tax included), and scopes the inputs (destination + page count). It is immediately distinguishable from siblings like send_fax and get_fax_status, and even discloses the eligibility-check side effect.

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 closing line '何も送信されません' clearly frames this as a pre-send lookup rather than an action, which is the key usage context against send_fax. However, it never explicitly names the alternative or states the when-not condition (e.g., 'use this before send_fax'), so it falls short of a 5.

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