check_contact_exists

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

No annotations are provided, so the description carries the full burden. It clearly communicates that this is a read-only check rather than a send operation, but it does not disclose the return format, error behavior, or whether it has no side effects beyond checking. The behavior is not contradicted, but the disclosure is incomplete.

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 a single sentence with no filler or repetition. The core purpose and usage timing are front-loaded, making it easy for an agent to parse quickly.

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 simple two-parameter check tool with a complete schema and a clear purpose, the description is largely sufficient. The main gap is that there is no output schema and the description does not state what the check actually returns, but an agent can reasonably infer a boolean or status result from 'check whether.'

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 parameters are already well documented. The description adds no additional parameter-level meaning, leaving the baseline score of 3 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 uses a specific verb ('Check') and a clear resource ('whether a phone number is registered on WhatsApp'), and it adds the intent 'before sending to it,' which distinguishes it from the send_* siblings. The tool's role as a pre-send verification step is immediately obvious.

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 phrase 'before sending to it' clearly indicates when this tool should be used, placing it in the workflow prior to send_text_message, send_image, or send_file. It does not explicitly name alternatives or exclusions, but the context is strong enough to guide selection.

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