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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the description's added value is the clarification that it never returns live subscriber status and reports uncertainty/rule evidence. This is useful behavioral context that goes beyond the annotations without contradicting them.

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 concise sentences with no filler. The primary purpose is front-loaded, followed by parameter guidance and a behavioral note. 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?

Given the presence of an output schema (as indicated in context signals), the description doesn't need to detail return values. It covers the main purpose, parameter usage, and a key behavioral limitation. The only minor gap is the lack of guidance on the 'detail' parameter, but overall it is sufficient for an agent to invoke the tool correctly.

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 0%, so the description must compensate. It explains the purpose of numberCountryCode and originCountryCode, but does not explain the 'detail' parameter (compact/full) or the format/constraints of phoneNumber. While phoneNumber is self-explanatory, the omission of detail is a gap given the low schema coverage.

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 action ('Analyze a number') and a specific resource ('using versioned numbering metadata'), and it distinguishes itself from siblings by clarifying it reports uncertainty and rule evidence, not live subscriber status. This clearly differentiates it from tools like format_international_phone_number or get_phone_capabilities.

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?

It provides concrete parameter guidance: national numbers need numberCountryCode, exit prefixes need originCountryCode. This tells the agent when to supply which parameter. However, it does not explicitly name alternative tools or state when to prefer this over others, leaving some inference to the agent.

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