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

No annotations are provided, so the description carries the full burden, and it does useful work by enumerating what the call returns (E.164, validity, line type, SMS capability, country, carrier, timezones) with no output schema to lean on. It omits operational traits such as auth requirements, rate limits, or whether this is a paid carrier lookup, which keeps it short of a 5.

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?

Three sentences, front-loaded with the verb, then returns, then the one non-obvious parameter rule. The return-field list is longer than strictly necessary but each item is informative, so little is wasted.

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?

With no output schema and no annotations, the description supplies exactly the two things missing: the return shape and the conditional for the optional parameter. Only operational context (auth, cost, limits) is absent, which is minor for a single-number read check.

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 0%, so the description must compensate, and it does: default_country is explained as an ISO code ('US', 'GH') used when no +country prefix is present, which is meaning the schema does not carry. The phone parameter itself is self-evident from its name.

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?

"Check one phone number" names a specific verb and resource, and the enumerated return fields make the scope unambiguous. It distinguishes itself from siblings by resource (phone vs domain, IP, email, contacts), though it never names an alternative explicitly.

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 gives one clear conditional: pass default_country when the number lacks a +country prefix. Beyond that it offers no guidance on when to use this tool versus validate_contacts (which may also cover phones) or when not to call it, leaving routing to inference.

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