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

Annotations only state that this is a non-idempotent, destructive, open-world write; the description adds substantive behavior beyond that: credit consumption, the fact that a real fax is transmitted, per-source size limits, one-time-use upload_id, and the shape of the outcome (sent, scheduled, or awaiting payment). That is well past what the structured fields declare.

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?

The cost and API-key requirement are front-loaded, and nearly every clause carries actionable content. It is a dense single block, though, and the base64 guidance somewhat duplicates the parameter description, so it is not maximally tight.

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 a 6-parameter, nested-object mutation tool with no output schema, the description covers input sources, limits, cost, geography, and even summarizes the return outcome. What is missing is any mention of failure modes or what happens when a fax cannot be delivered, which would round out an otherwise complete definition.

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 baseline would be 3, but the description adds genuine meaning on top: the mutual exclusivity rule ("DOCUMENT (exactly one)"), why base64 is discouraged, the role of document_name on the transmission report, and the max-700 KB base64 ceiling. It stops short of fully documenting the nested "file" object beyond the schema's own note.

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") plus the immediate cost ("1 prepaid credit"), and it names create_upload_link as the way to obtain an upload_id, which separates it from its siblings. An agent can tell instantly that this is the transmission tool and not a lookup tool like get_fax_price or get_fax_status.

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?

It spells out prerequisites (credits API key sfx_..., a direct public HTTPS PDF link), constraints (10 MB, 100 pages), supported geography, and an explicit exclusion (premium-rate numbers are not supported). It also routes the agent between the four document sources and points at create_upload_link when an attachment isn't available as "file", which is real alternative-selection guidance.

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