phone_lookup

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

Description adds significant behavioral details beyond annotations: carrier omission in MNP-restricted regions, the need to read carrier_status field to interpret carrier presence, and the exact return structure. No contradiction with 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?

Description is reasonably concise and well-structured, front-loading purpose and constraints, then adding details. Every sentence adds value, though slightly long.

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?

Description covers all aspects: validation, analysis, format, rate limits, companion tools, return structure, and critical behavioral nuance about carrier omission. Given the output schema exists, this is complete.

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%. Description adds value by explaining E.164 format with examples and common mistakes. This goes beyond the schema's basic description.

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?

Description clearly states tool validates and analyzes phone numbers for country, region, carrier, line type, timezone, and formats. It mentions use for verifying legitimacy and fraud detection, and distinguishes from sibling tools by naming companion tools (username_lookup, email_disposable).

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?

Description specifies when to use (verify legitimacy, detect fraud), prerequisite (E.164 format), and rate limits (30/hr Free, 500/hr Pro). It does not explicitly state when not to use, but provides companion tool context for alternative uses.

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