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

Annotations already declare non-read-only, idempotent, non-destructive and open-world behavior, so the safety profile is covered structurally. The description adds one genuinely useful constraint (the 8-participant capacity), but says nothing about what happens when the group is already full, whether existing participants are preserved, or what errors/auth failures look like.

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?

A single front-loaded sentence with the action, target and limit in order, and no filler. Every clause earns its place.

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 4-parameter mutation tool with no output schema, the description is adequate but thin: it omits permission requirements, capacity-exceeded behavior, and whether additions are atomic. Annotations carry the safety profile, so these gaps are real but not fatal.

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 75% and the schema already documents phone_number_id and profile_id; the description only restates the 'at most 8' constraint that the schema's phone_numbers field also carries. It adds no format or source detail 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?

States a specific verb (Add) and resource (phone numbers to a WhatsApp group) and quantifies scope with the 8-participant cap. It is immediately distinguishable from the sibling whatsapp_group_participants_remove 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?

There is no guidance on when to use this versus alternatives (e.g. invite-link creation or join-request approval), and no stated prerequisites such as needing group admin rights or an existing group. The agent must infer all of this from the name.

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