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

Annotations already cover readOnlyHint, idempotentHint, and destructiveHint, reducing the burden on the description. The description adds substantive behavioral context beyond annotations: the external IPQualityScore API dependency, the requirement to pass a caller's own _apiKey, and the output dimensions (fraud score, line type, carrier, region, risk flags). This is valuable given there is no output schema.

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 dense, purposeful sentences: the first front-loads the core function and output, the second gives a concrete invocation example, and the third states the required API key. Every sentence earns its place with no filler or redundancy.

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 covers purpose, key output facets, the external API key requirement, and an example call. It does not specify the exact response shape or mention rate limits/free-tier caveats, but it provides enough for an agent to select and invoke the tool correctly, especially with annotations carrying the safety profile.

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%, with each parameter (phone, _apiKey, country) already documented with examples and purpose. The description repeats the phone/country example but adds no new meaning 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 states a specific resource (phone) and distinct actions: validation, active-status confirmation, fraud score (0-100), and flags for VOIP/prepaid/risky numbers with line type, carrier, and region. This clearly differentiates it from sibling tools like check_email, check_ip, and check_url, which share the check_* pattern but target different resources.

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 'use when you need phone number validation and fraud checking' but provides no explicit when-to-use/when-not-to-use guidance or named alternatives. It does not reference sibling tools like check_email or check_ip, so an agent must infer the appropriate selection from the resource type alone.

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