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

Annotations already declare non-readOnly, idempotent, non-destructive, openWorld. The description adds real behavioral context beyond that: auto-registration happens on connect with a default PIN, and already-registered numbers return already_registered: true. It stops short of describing error/auth failure behavior, so not a 5.

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: purpose first, then the automatic-behavior caveat and the trigger conditions, then the idempotent return. Every sentence carries information and nothing is redundant.

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?

No output schema exists, but the description discloses the key return signal (already_registered: true) and covers prerequisites and triggers. It could say more about failure modes (invalid PIN, unverified number), but is largely complete for a 3-param tool.

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% and the schema already documents all three params, including that pin defaults to Postproxy's default PIN. The description's restatement of the 6-digit PIN adds no meaning beyond the schema, so 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?

States a specific verb+resource: register a phone number with the Cloud API using a 6-digit two-step verification PIN. This clearly distinguishes it from sibling whatsapp_number_verify (code verification) and whatsapp_number_request_verification_code.

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 says when NOT to bother (Postproxy does it automatically on connect with a default PIN) and gives concrete triggers for calling it: when number_info reports a registration_warning, or after the PIN changed. This is explicit when/when-not guidance tied to a named sibling tool.

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