Look up a phone number

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

No annotations are provided, so the description carries full burden. It clearly discloses limitations: area-code level only, no identity or carrier, and spoofing warning. This gives the agent important behavioral context. It does not describe side effects, but as a read-only lookup, that is acceptable.

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 two sentences: first sentence states the action and return values, second sentence clarifies limitations. It is front-loaded, every sentence adds value, and there is no fluff. Perfectly sized for quick understanding.

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 no output schema, the description enumerates all return fields (validity, E.164, area code, region, time zone, local time) and emphasizes the area-code-only limitation. With a simple single-parameter tool, this fully covers the agent's needs for correct invocation and understanding of results.

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?

The schema describes the parameter with an example, achieving 100% coverage. The description adds value by specifying the geographic scope ('US/Canada (NANP)') and connecting it to the overall validation purpose, which goes beyond the schema description.

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 verb 'Parse and validate' and the resource 'US/Canada (NANP) phone number', and lists specific return values. It distinguishes this tool from siblings by explicitly noting it provides area-code level info only — not caller identity, exact location, or carrier — which sets it apart from sibling tools like lookup_carrier or check_number_reputation.

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 implicit usage guidance by stating what the tool does NOT do (identity, exact location, carrier) and mentions that caller ID can be spoofed, helping the agent decide when not to use it. However, it does not explicitly reference sibling tools or give direct comparisons like 'use this for validation, use lookup_carrier for carrier info'.

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