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

Goes well beyond annotations: irreversibility ('cannot be recalled'), non-idempotency ('every call sends a new one'), the failure mode of WhatsApp (silently dropped by Meta), and the cost model (1 credit, charged only on success). These are exactly the operational facts an agent needs before invoking a real outbound send.

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?

Four short sentences, front-loaded with the core action and default, then consequences and cost. No filler; each sentence carries a distinct operational fact.

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, so the description enumerates the return fields (message id, channel, recipient, sender number, status). Combined with the loss/cost warnings and annotations, an agent has everything needed to call it 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?

Schema description coverage is 100%, so the schema already documents the E.164 format, enum values, and default. The description restates the channel default and opt-in rule rather than adding new syntax or constraints, 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?

Opening line states a specific verb (send) plus the two resources (SMS/WhatsApp) and immediately clarifies the default channel, so it is unambiguous against siblings such as emails or tts. An agent can distinguish it without opening the 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?

Gives a direct when-to-use rule ('text a number the user gave you') and explains the conditions for the WhatsApp channel (recipient must have messaged within 24 hours). It does not explicitly mention alternative tools (e.g., emails for non-phone delivery), 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.