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

Annotations already declare destructiveHint, non-idempotent, and open-world behavior, but the description adds critical process semantics they cannot convey: payment is required via checkout_url and sending is asynchronous (auto-sent only after payment), plus format/size/page limits and single-use upload_id. It stops short of describing failure modes or retry behavior.

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-loads the core action and return value before enumerating document options and constraints. It is a dense single block but every clause carries operative information; only the parenthetical detail is slightly heavy.

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 correctly explains the return value (checkout_url) and the payment-then-send flow, and it covers the mutually exclusive input sources for a 9-parameter tool. Missing only error/rejection handling details, which keeps it from a 5.

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, but the description adds real value by stating that exactly one of upload_id/file/document_url/document_base64 must be supplied and by attaching constraints (700KB, 10MB, 100 pages, Japanese-time scheduling) that the schema does not enforce. This is meaningfully beyond field-by-field docs.

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 (send a FAX) and the concrete return value (a Stripe checkout_url in JPY), and distinguishes itself from the sibling create_upload_link by explaining that upload_id originates there. An agent can tell this is the send-and-pay entry point rather than a pricing or status query.

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 for each document source option (upload_id for device files, file for ChatGPT attachments, document_url for public PDFs, document_base64 for small files) and mentions the scheduling path. It does not explicitly say when to prefer get_fax_price or get_fax_status, so routing among all siblings is only partially covered.

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