is_on_whatsapp

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

Annotations already indicate read-only, non-destructive, idempotent, open world. The description adds value by explicitly stating no notification to queried users and no chat side effects, plus outlining the return format, which is not present in annotations.

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?

Three sentences with no redundancy. First sentence states core functionality and side effects, second gives usage recommendation, third describes return format. Information-dense and well-structured.

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 simple single-parameter read-only tool, the description covers purpose, behavior, usage context, and return format. No output schema exists, but the description compensates by describing the JSON structure.

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?

The schema has 100% coverage for the 'phones' parameter. The description provides essential formatting details (digits only, no + or spaces, example) and validation rule (non-empty), adding significant meaning beyond the schema's basic description.

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 clearly states it queries WhatsApp servers to check phone number registration, specifies that users are not notified, and distinguishes itself from sibling tools like send_message by focusing on existence check.

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?

Explicitly recommends use before send_message when only a phone number is available and confirmation of contact existence is needed. However, it does not mention when not to use it, such as when the contact is already known, but the context is largely clear.

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