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

Annotations already declare the safety profile (destructive, non-idempotent, open-world write). The description adds real context beyond that: credit consumption (1 credit per fax, up to 100 pages regardless of page count), upload_id being single-use, and the 10MB/100-page file limits and scheduling window.

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?

Front-loaded with the core action and credit cost, followed by file-source options and limits. It is a single dense paragraph, so it is efficient but lacks visual structure and slightly compresses a lot into one block.

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 an 8-parameter mutation tool with no output schema, the description covers document sources, size/page constraints, credit cost, and scheduling adequately. It does not describe the success/failure response the caller should expect, which is a modest gap given no output schema exists.

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 the baseline is 3; the schema already documents each parameter individually. The description adds genuine meaning beyond the schema by stating the four document sources are mutually exclusive ('いずれか1つ'), which no individual parameter description conveys.

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 and resource — '実際にFAXを送信します' (actually sends a FAX) — and the '実際に' framing implicitly contrasts with the preparatory/query siblings (create_upload_link, get_fax_price, get_fax_status). It never names those siblings explicitly, so it falls short of full sibling differentiation.

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?

Gives clear context on the four mutually exclusive document sources and which to prefer ('通常は create_upload_link をお使いください'), plus that scheduled_at defers sending. It does not state when to call get_fax_price first or get_fax_status afterward, so no explicit when-not guidance.

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