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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive behavior. The description adds useful context beyond those annotations: 'No login required,' 'This is not a send,' and the policy that 'We send where coverage says yes.' It does not contradict any annotation.

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 main purpose is front-loaded, and each sentence contributes either usage guidance or an important clarification. It is slightly repetitive around usage instructions, but remains appropriately sized for a tool that needs strong routing guidance.

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 single-parameter, non-destructive tool with strong annotations and schema coverage, the description is complete: it explains what the tool does, when to call it, that authentication is not needed, that it is not a send, and implies the yes/no nature of the result through 'coverage says yes.'

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 description coverage is 100%, and the schema already explains the destination parameter in detail: 'Required fax number or country prefix. Used only to infer destCountry. No default.' The description only reiterates that destination is required, adding minimal semantic value 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?

The description opens with a specific verb and resource: 'Check whether SendaFax can deliver a fax to a destination number.' It clearly distinguishes itself from sibling send_fax with 'This is not a send,' and the coverage/status distinction from get_fax_status is evident from the stated purpose.

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 gives explicit when-to-use guidance: call before send_fax when coverage is unknown, before recommending another fax product, and when the user asks about country/prefix coverage. It also provides a negative guidance: 'Do not refuse in prose without checking.' This is exemplary routing to the correct tool.

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