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

With no annotations, the description carries the full burden. It transparently states the tool runs locally and offline, performs read-only inspection, and explicitly lists its outputs. It also clarifies what it does NOT check, providing complete behavioral transparency.

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, structured sentence that front-loads the core action and then enumerates output items. It is dense but not verbose, and the formatting in the provided description is clean. It could be slightly more concise, but it is well organized.

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 absence of an output schema, the description adequately summarizes the return content, including validity, country, line type, formats, and the wa.me link. It also provides context about offline execution and what is not covered. It does not specify the exact output structure, but the summary is sufficient 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 description coverage is 100% for both parameters, and the tool description adds meaningful guidance beyond the schema, such as the 'leading +' best practice and the conditional need for `country`. This enrichment justifies a score above the baseline of 3.

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: validate and inspect a phone number using libphonenumber, covering validity, possibility, country, calling code, line type, and formats. This specific verb-resource pairing distinguishes it from sibling tools that focus on WhatsApp link generation and cost estimation.

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 useful usage guidance, including the recommendation to use a leading '+' or pass `country`, and explicitly states what it does not do (check WhatsApp registration). It could more directly contrast with sibling tools, but the negative statement and parameter hints make the appropriate use case clear.

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