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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds genuine context beyond that: the cost of 1 caller-ID credit per call, which affects invocation decisions. It does not state whether credits are consumed on a not-found result or what the response contains on failure.

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 sentences with zero waste; the core purpose is front-loaded before the cost note and use case. Every clause earns its place.

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 tool with no output schema and annotations covering the safety profile, the description supplies purpose, cost, and use case. The remaining gap is failure/unknown-number behavior and whether a credit is still charged, which an agent would need for reliable invocation.

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% and the single parameter documents both E.164 and local formats, so the schema does the heavy lifting. The description adds no format or constraint detail beyond what the schema already provides, making the baseline 3 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?

The description names a specific verb and resource (reverse phone lookup retrieving CNAM for a phone number), which is unambiguous and clearly separable from siblings like validate_phone or check_whatsapp. It stops short of explicitly naming those siblings as alternatives, so it is clear but not sibling-differentiating.

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?

"Use to identify who owns a number" gives an implied use case but no explicit when-not guidance or named alternatives (e.g., validate_phone for validity vs. this for ownership). The agent must infer selection among the many phone-related siblings.

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