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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint=false, so the safety profile is covered; the description adds that nothing is transmitted and, more usefully, that it performs destination eligibility validation and rejects mobile/0990/0570 numbers. It does not mention rate limits or whether pricing can change, keeping it out of 5 territory.

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 short sentences, each earning its place: what is returned, the extra eligibility signal, and the no-side-effect guarantee. Front-loaded with the primary outcome and free of filler.

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 carry return-value burden and does so reasonably: price plus supportability, with currency and tax basis specified. Given the flat 3-parameter schema and strong annotation coverage, this is close to complete; only the return shape/format of a rejection is unspecified.

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 description coverage is 100%, so the baseline is 3. The description restates the destination/page-count inputs and adds the currency/tax semantics (日本円・税込) and the unsupported-number rule, but says nothing about promo_code or the accepted number formats that the schema already covers.

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?

Specific verb+resource: it returns the FAX price in JPY (tax included) derived from destination and page count, and additionally reports whether the destination is supported. That is clearly distinct from send_fax, though the description never names the sibling it differs from, so it stops short of a 5.

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?

Usage is implied rather than stated: an agent can infer this is the pre-send pricing/eligibility check, reinforced by '何も送信されません'. There is no explicit 'use this instead of X when Y' routing against send_fax or get_fax_status, so it is only minimum-viable guidance.

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