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

Annotations already indicate readOnlyHint=true, so safety is covered. The description adds meaningful behavioral context on top: it discloses that the tool calls Loqate, requires an API key, and returns specific outputs (confidence, number type, carrier, and a recommendation). This is valuable information beyond the annotations.

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-loads the primary action, and packs in return values plus authentication requirements without any filler. Every sentence contributes useful information.

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 tool with no output schema, the description adequately explains what the tool returns and the key prerequisite (API key). Optional parameters and defaults are left to the schema, which is acceptable since the schema is fully descriptive. The main gap is lack of guidance on when to prefer this tool over sibling verification tools, but that is more of a usage-consideration gap than a completeness gap for making a successful call.

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%, so the structured schema already documents all parameters thoroughly. The description adds a small clarification about passing the API key via the 'key' field, but it does not substantially enrich the meaning of parameters such as policy or country beyond what the schema provides.

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's function: verifying a phone number via Loqate, and lists the key outputs (confidence, number type, carrier, recommendation). It is specific and unambiguous, but it does not explicitly differentiate itself from sibling tools like verify_contact or verify_email, relying mostly on the resource name 'phone' to signal distinctness.

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 implies usage when phone verification is needed and mentions the API key requirement, but it does not provide explicit guidance on when to choose this tool over alternatives such as verify_address or verify_contact. No when-not-to-use conditions or exclusions are given.

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