is_on_whatsapp

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

Annotations already indicate readOnlyHint, destructiveHint, idempotentHint, and openWorldHint. The description adds minimal behavioral context beyond 'check registered numbers' and does not explain what the output looks like (e.g., list of booleans or registered numbers) or any other behavior.

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 exceptionally concise: two sentences that front-load the purpose and then provide a key constraint. Every word adds value with no redundancy.

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?

Given the low complexity (one parameter, no output schema), the description covers purpose and input format but lacks details on return value, error conditions, or how the tool handles invalid inputs. It is functional but not fully complete for an agent.

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 0%, so the description must compensate. It adds that phone numbers must be 'digits only (no +)', which provides format guidance. However, it does not clarify that the parameter is an array of strings or specify expected format (e.g., country code), leaving some ambiguity.

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 clearly states the tool's purpose: 'Check which phone numbers are registered on WhatsApp.' This is a specific verb and resource, and it distinguishes itself from siblings like 'search_contacts' or 'list_chats'.

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 only an input formatting constraint ('digits only, no +') but no guidance on when to use this tool versus alternatives. No mention of when not to use it or any context about its role relative to sibling tools.

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