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

Goes well beyond the annotations (destructiveHint/openWorldHint/idempotentHint) by disclosing the payment gate ('fax is sent after the user pays'), quota consumption, real-world transmission, one-time use of upload_id, size ceilings (10 MB link, 700 KB base64), country coverage, and what the result reports (sent/scheduled/awaiting payment).

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 action and the payment branches, and dense with useful detail, but the document-source enumeration restates schema parameter descriptions and adds length. It is efficient for its complexity though 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?

With no output schema and an 8-parameter mutation tool, the description still covers auth/payment model, input constraints, geography, and return-value meaning ('states whether the fax was sent, scheduled or is awaiting payment'), leaving nothing essential for a correct call.

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 already 100%, yet the description adds real meaning: the mutual exclusivity of the four document sources ('exactly one'), size limits per source, and that document_name is unnecessary with upload_id or file. It stops short of describing email or scheduled_at semantics, which the schema 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?

Opens with a specific verb+resource ('Send a PDF document to a fax number') and immediately names the alternative path (create_upload_link) for obtaining a document, which cleanly separates it from its siblings get_fax_status, get_fax_price and list_fax_history.

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?

Explains the two operating modes (pay-per-fax checkout link vs. subscription key for immediate send), the constraint that exactly one of upload_id/file/document_url/document_base64 must be supplied, the geographic and premium-rate exclusions, and when to fall back to create_upload_link. An agent can pick the correct invocation path without inference.

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