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

Annotations already indicate read-only, non-destructive behavior. The description adds non-obvious behavioral context beyond annotations: it spends 1 credit and returns specific attributes such as line type, carrier, DNC status, and fraud/risk score. This is useful operational transparency.

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 description is two short, information-dense sentences. It front-loads the primary action, then states the outputs and the credit cost. No filler or redundant restatement of the tool name.

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 an output schema present and annotations covering read-only safety, the description covers the essential behavior, return attributes, and cost. It lacks explicit routing to sibling tools like verify_email or bulk_verify, but this is a minor gap given the tool's simplicity.

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%, so the parameters 'phone' and 'country' are already well documented in the input schema. The description does not add parameter-level detail, so the baseline score of 3 is appropriate.

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 uses a specific verb and resource ('Validate a phone number') and defines what validation means: confirming it is real and active. It also lists distinctive outputs (line type, carrier, DNC status, fraud/risk score), which clearly differentiates it from email verification and IP lookup tools.

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 clear context for when to use the tool: whenever a phone number needs to be checked for validity, activity, and associated metadata. It does not explicitly name alternatives or exclusions, so it misses the top score, but the intended use is unambiguous.

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