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

Annotations already signal a non-readonly, destructive, non-idempotent external action. The description adds valuable behavioral context beyond that: the action is scoped to the user's own account, requires an existing chat, and is gated on explicit user approval, which mitigates the risk of unsolicited sends.

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?

The description is three short sentences with no filler. The core action is front-loaded, followed by the prerequisite for chat_id and the approval guardrail, making it easy for an agent to parse in order.

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 side-effecting send action, the description supplies the critical missing context: how to resolve an unknown chat_id and when sending is permitted. It leaves implicit the fallback to whatsapp_start_conversation for a new chat and does not describe the return value, but the tool is otherwise simple enough to call correctly.

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?

The input schema already documents chat_id and account_id in detail, and the description adds no new parameter-level meaning beyond saying chat_id must refer to an existing chat. With schema coverage at 67% and text being self-explanatory, the baseline of 3 is appropriate.

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: 'Send a WhatsApp message in an existing chat from the user's own account.' It clearly distinguishes this tool from related siblings like whatsapp_start_conversation by limiting it to existing chats, and the provider-specific name prevents confusion with generic messaging_send_message.

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 description provides explicit operational guidance: call whatsapp_list_conversations first if chat_id is unknown, and only send after the final message is explicitly requested or approved. It does not explicitly mention when to use whatsapp_start_conversation for new chats, so it stops short of full alternative routing.

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