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

No annotations are provided, so the description carries the full disclosure burden. It implies a non-mutating lookup, but says nothing about return shape (boolean vs. object), whether a connected device/session is needed, rate limits, or quota costs on WhatsApp's side.

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, front-loaded sentence with no filler. Nothing in it is redundant with the tool name or title.

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?

The tool is simple: one required string parameter, no nested objects, and no output schema. Given this simplicity, the description is adequate, but without an output schema the description should say what a successful check returns (a boolean, a status object, or something similar).

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?

There is a single parameter with 100% schema description coverage, so the schema already carries the format detail ('E.164 format'), minLength, and maxLength. The description adds nothing beyond what the schema states, which matches the baseline for high coverage.

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: 'Checks if a given phone number is associated with an active WhatsApp account.' This is far more than a restatement of the name. However, it does not distinguish itself from the sibling validatePhoneNumbers, which an agent could easily confuse with this one.

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?

There is no explicit guidance on when to use this tool versus the sibling validatePhoneNumbers, or versus contact-lookup tools like manage_whatsapp_contacts. The agent must infer usage from the name alone.

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