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

Annotations already establish readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds genuinely useful non-schema behavior: flat 1 credit per fax capped at 100 pages, no API key required, coverage of USA/Canada/Australia/NZ plus 39 countries, and that premium-rate numbers are unsupported.

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?

A single dense sentence, front-loaded with the core capability (support check plus cost) before the qualifiers. The parenthetical asides are informative rather than padded, though the sentence is long enough that scanning suffers slightly.

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 one-parameter read-only tool with no output schema, the description covers cost, coverage, auth, and unsupported cases, and 'how many credits it costs' implies the return shape. The only gap is that it never explicitly states the return format (support flag plus credit amount).

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% with a single well-documented fax_number parameter (E.164 with examples), so the schema does the heavy lifting. The description contributes only the constraint that premium-rate numbers are rejected, which is a useful validity note but not syntax or format detail beyond the schema.

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: it checks whether a fax destination is supported and returns its credit cost. This is clearly distinguishable from siblings like send_fax (which sends) and get_fax_status (which tracks an existing fax), since this is a pre-flight lookup.

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?

The description implies the natural use case (validate support/cost before sending) by naming coverage and pricing, but never explicitly says when to call it versus send_fax or get_fax_status. No exclusions or alternative routing are stated, leaving the agent to infer the pre-send checking role.

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