Validate phone numbers

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

Annotations already signal read-only/idempotent/safe; the description adds non-redundant details: batch limit, libphonenumber data, validity/type/form outputs, and the country fallback for numbers without '+'. It does not repeat annotation flags or contradict them.

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?

Two dense sentences with no filler; the first sentence front-loads scope and capability. The second is a fragment but communicates country semantics efficiently. Slightly run-on structure prevents a 5.

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 read-only validation tool with an output schema and annotations, the description covers input limits, optional country semantics, and result categories. It does not need to explain return values because an output schema exists; minor omissions such as exact error behavior keep it from a 5.

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 documents country as 'default country, e.g. GB' but the description enriches it with 'ISO code' and the key condition 'for numbers written without +'. The numbers array is left implicit but is self-evident from the description; more direct parameter-level detail for numbers would be ideal, yet coverage is adequate.

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 ('validates'), exact resource (phone numbers), a hard limit (up to 100) and data source (Google's libphonenumber). It also enumerates output dimensions, making it easy to distinguish from sibling check_addresses/check_emails.

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?

No explicit when-to-use or alternative routing is given; usage must be inferred from the tool name and resource type. The description does provide sufficient context that a phone-number validation request should be routed here, but it never states exclusions or how this differs from check_addresses/check_emails.

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