Validate a phone number: is it a well-formed E.164 number, and what country calling code and region does it carry? Pure format + country check, no network call, no message sent. Free. For the authoritative carrier and line type (mobile / landline / VoIP), use the paid phone-intel tool.
| Name | Required | Description | Default |
|---|---|---|---|
| number | Yes | Phone number in E.164 (+14155552671) or loosely formatted |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description must carry the full behavioral burden — and it does disclose the important traits: no network call, no message sent, and that the operation is free. It stops short of describing error behavior for malformed input or any rate limits, which is the only remaining gap.
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 compact sentences with the core capability and return values front-loaded, followed by the cost and no-side-effect claims and the alternative-tool routing. No filler or restatement of the name.
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 single-parameter, read-only validation call with no output schema, the description supplies what is needed: the accepted input form, what is returned (country calling code and region), the absence of side effects, and the cost. Nothing an agent needs to call it correctly is missing.
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 the single 'number' parameter, so the schema already documents the E.164-or-loosely-formatted accepted input. The description adds only marginal meaning beyond that baseline, so a 3 is 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?
States a specific verb and resource ('Validate a phone number') and enumerates the exact checks performed: E.164 well-formedness plus country calling code and region extraction. The scope is precise enough that an agent can distinguish it from any carrier/line-type lookup without opening the schema.
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?
Explicitly routes the agent: this tool is for 'pure format + country check', and for carrier/line type (mobile / landline / VoIP) the description names a distinct paid phone-intel alternative. Both the when-to-use and the when-to-use-something-else conditions are stated, not inferred.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
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