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

Annotations already declare readOnlyHint=false, idempotentHint=false, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description usefully adds the precondition that the number must not yet be verified and that the call triggers an external message. However, it doesn't address the implications of idempotentHint=false (repeated calls send additional codes) or any rate limiting, which would materially affect invocation.

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?

Two tight sentences with the action and precondition front-loaded and the follow-up step immediately after. No filler, no repetition of the tool name, and nothing an agent must skip past to act.

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 still tells the agent what the call does (dispatches a code) and what to do next, so the workflow is closed. It omits repeated-invocation behavior and any indication of what the response contains (e.g. code length or expiry), leaving a small gap for a non-idempotent outbound 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 description coverage is 100%: method (SMS/VOICE enum), language locale, profile_id, and phone_number_id are all documented in the schema itself. The description's 'by SMS or voice call' merely restates the method enum without adding syntax, defaults, or constraints beyond what the schema already provides, so the baseline of 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?

Names a specific actor and action ('Ask Meta to send a one-time verification code') plus the target resource (a not-yet-verified phone number) and the delivery channels (SMS or voice call). It also names the follow-up sibling, whatsapp_number_verify, so an agent can separate this from whatsapp_number_register without opening either schema.

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?

States a clear precondition ('not-yet-verified phone number') and an explicit next step ('Follow with whatsapp_number_verify'), which gives real sequencing context. It stops short of naming alternatives or exclusions, e.g. what to do if the number is already verified or how this differs operationally from whatsapp_number_register.

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