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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds genuinely non-redundant behavior: no message or OTP is sent, billing is '1 WA credit per successful call' (implying failed calls are free), and the enrichment data returned. It stops short of covering failure modes or invalid-number behavior.

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?

One dense paragraph, front-loaded with purpose, then side-effect clarification, cost, and return fields in descending priority. Every sentence carries information an agent needs; nothing is padding.

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 no output schema, the description correctly compensates by naming the key return fields (numberstatus, businessnumber, carrier/line type/country) and disclosing cost. It is close to complete, missing only error/invalid-number behavior and a nod to the batch sibling for bulk work.

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%, with both phone_number (E.164 or local) and country_code already fully documented in the schema. The description adds no syntax or format detail beyond that, so the baseline 3 applies.

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 names a specific verb and resource ('Check whether a phone number has an active WhatsApp account') and immediately scopes it further with the Business-account variant. It is clearly distinguishable from siblings such as validate_phone and whatsapp_batch_submit, which do number validation and batch submission respectively.

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 clause 'without sending any message or OTP' tells the agent this is a non-invasive check, which is useful implied usage context. However, no alternative is named (validate_phone, whatsapp_batch_submit, whatsapp_batch_status) and there is no explicit when-to-use/when-not-to-use guidance, leaving routing to inference.

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