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

Goes well beyond the annotations: it discloses that starting to type opens the conversation, marks unread messages as read, appears only after a short pause, expires on its own, consumes no quota, and returns payment_required: feature_not_available for gated plans. These are non-obvious side effects an agent could not infer from readOnlyHint=false and destructiveHint=false alone.

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?

Well structured and front-loaded: purpose, recipient constraint, timing rationale, cleanup behavior, side effects, then plan gating. A few sentences are redundant with the schema text (the group plan caveat is repeated verbatim from the group_id description), which costs some efficiency but not clarity.

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?

Despite five parameters and no required ones, the description supplies everything needed to call it correctly: recipient selection, the default is_typing behavior, lifespan, side effects on unread state, quota impact, and the exact failure mode for unentitled plans. Return values are covered by the output schema, so nothing material is missing.

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%, so the schema already documents each parameter; the description reinforces the exactly-one-of number/group_id/contact_id constraint and adds meaning about the absent duration argument ("WhatsApp decides how long it lives"). The clarification that is_typing=false only clears the indicator and reads nothing is a genuine semantic addition beyond 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?

The description names a specific verb and resource ("Show 'typing…' to a recipient") and states the intent context ("while you prepare an answer"). It is immediately distinguishable from sibling send tools such as send_text or send_media, which deliver content rather than a transient presence state.

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?

It gives a concrete when-to-use condition ("before a reply that takes a few seconds to compose") and a when-not/cleanup rule ("pass is_typing=false only if you decide not to answer after all"). It does not explicitly contrast with send_text/reply_to_message or say what to do if the feature is unavailable beyond not retrying, so it falls 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.