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

Annotations cover the safety profile (readOnly=false, destructive=false, idempotent=true, openWorld=true), and the description adds value beyond them by disclosing the cross-number side effect of force_transfer and the format validation rules. It stops short of describing error behavior or permissions, but the transfer consequence is the key non-obvious trait and it is surfaced.

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 tight sentences, front-loaded with the action and resource, followed by the format constraint and then the conditional transfer hint. No filler or repetition.

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 4-param setter with no output schema, the description covers purpose, format rules, and the notable transfer scenario. It leaves error handling (e.g. username already taken) and required permissions unstated, which is a minor gap rather than a blocking one.

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 only 50%, but the description compensates well: it spells out the username character rules (letters, digits, periods, underscores, must contain a letter, no leading/trailing/consecutive periods) and explains transfer_action's force_transfer value, which the enum alone does not. profile_id and phone_number_id are already documented in 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?

States a specific verb pair ('claim or change') plus the exact resource ('the WhatsApp username of a phone number'). This cleanly separates it from the sibling whatsapp_username_get, whatsapp_username_delete and whatsapp_username_suggestions, and even captures both create and update cases in one phrase.

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?

Provides a concrete conditional: use transfer_action: force_transfer when a username is already held by another number in the same business. That is real when-to-use guidance, though it does not name alternative tools or state when NOT to use this one.

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