Verify a phone number against carrier reference data

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

With no annotations, the description carries full responsibility and does well: it discloses that line type is not reported (a common expectation), that Premium mode is not assumed, and warns against using output for SMS deliverability. This goes beyond the basic 'validates' claim and informs the agent of significant behavioral limitations.

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, front-loaded with the primary purpose, followed by critical limitations. Every sentence adds value, and it is compact without any filler.

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 one-parameter verification tool with no output schema or annotations, the description covers validity checks, standardization, and critical exclusions. It stops short of specifying the exact return structure or error behavior, but given the simplicity, it is adequately complete for an agent to call it correctly.

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 single parameter phoneNumber is fully covered by the schema description ('any common format'). The tool description adds no additional parameter constraints or format details beyond the schema, so the baseline of 3 applies.

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 verifies phone number validity and returns a standardized version, with a specific verb and resource. It also distinguishes itself from siblings (verify_email, verify_postal_address) by focusing on phone, and explicitly notes a limitation (no line type detection). This allows an agent to select it correctly among siblings.

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 a clear context of use: validating and standardizing phone numbers. It includes an explicit exclusion: 'Do not use this tool's output to infer SMS deliverability,' which prevents misuse. However, it does not directly reference alternative tools for other data types, so it leaves a small gap in routing to siblings.

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