Check if numbers are on WhatsApp

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

Annotations already declare read-only, idempotent, non-destructive, and open-world behavior. The description adds useful context beyond them by noting the recipient is not notified and that a JID is returned for each number, though it omits edge behavior for unregistered numbers and rate limits.

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, front-loaded with the purpose, followed by usage guidance and a safety note. Every sentence earns its place with no redundancy.

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 read-only batch check, annotations and schema cover most needs, and the description adds the JID return and non-notification behavior. It does not fully specify the response shape for numbers not registered on WhatsApp, which is a minor gap without an output schema.

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%, so the schema already documents the numbers array format, country-code requirement, max 50, and optional instance_id. The description adds only that one or more numbers are accepted, which is minor 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 states a specific verb and resource: checks whether phone numbers are registered on WhatsApp and returns the JID for each. It also distinguishes the tool from sending siblings by explaining its role before sending.

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?

It gives clear usage context: use before sending to a number you are not sure about to avoid failed sends. It does not explicitly state when not to use it or name an alternative tool, but the guidance is actionable.

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