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

Annotations only declare the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false); the description adds substantive traits beyond that: 30-minute expiry, one-time use, format and 10MB size limits, and the explicit statement that creating the link sends nothing and costs nothing. These are exactly the constraints an agent needs before invoking.

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?

Purpose and limits are front-loaded in the first sentence, and the workflow note follows. The clause about smartphone/PC upload and camera capture is marketing filler that does not help an agent decide or invoke, slightly diluting an otherwise tight definition.

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 still conveys the key return artifact (upload_url) and the follow-up handle (upload_id) needed for the next call. Combined with format/size/expiry constraints and the no-side-effect note, an agent has everything required to use this tool correctly.

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?

The tool takes zero input parameters, which is the baseline-4 case. The description does name the runtime artifacts (upload_url, upload_id) that flow through the tool, adding useful meaning about what the call ultimately produces.

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 + resource (creates a secure one-time upload URL) and scopes it precisely (FAX documents, PDF/JPG/PNG, max 10MB). It also positions the tool in the workflow by naming send_fax and the upload_id handoff, so an agent can distinguish it from get_fax_price/get_fax_status/send_fax without opening any schema.

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?

Clear sequencing guidance: create the link first, have the user upload to upload_url, then pass upload_id to send_fax. It also clarifies that this step is side-effect free (nothing sent, no charge). It does not explicitly state when NOT to use it or alternatives, which keeps it 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.