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

Annotations include readOnlyHint=true and openWorldHint=true, establishing the read-only nature. The description adds behavioral details such as validation and the specific return fields (country, carrier, line type, E.164 format), which goes beyond annotations. It does not disclose error handling or format limitations, but the annotations already cover the safety profile.

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 compact: two sentences, with the first sentence stating the core function and output, and the second providing usage guidance. No redundant or filler content.

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?

Given the tool's simplicity (one parameter, no output schema), the description adequately covers purpose, usage, and expected output fields. It lists the key return values, making the tool understandable without an output schema. Minor gaps like potential error behavior or supported regions do not detract significantly for such a lightweight tool.

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 input schema has full coverage (100%) with a description for the 'phone' parameter ('Phone number to validate'). The tool description adds minimal extra semantic value about the parameter itself, only hinting at E.164 format in the output context. Therefore, baseline 3 is appropriate since the schema does the heavy lifting.

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: 'Parse and validate a phone number, returning country, carrier, line type, and E.164 format.' This specific verb+resource combination sets it apart from sibling lookup tools like lookup_email or lookup_ip.

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 provides clear usage context: 'Use when normalizing user-submitted phone numbers or verifying contact data.' This tells the agent when to invoke this tool, though it does not explicitly mention alternatives or exclusion scenarios.

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