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

Annotations already provide readOnlyHint=true and openWorldHint=true, covering safety and external interaction. The description adds the per-call cost (40P), which is a meaningful behavioral detail. However, it does not disclose the return format or behavior on invalid inputs, leaving a gap beyond what annotations convey.

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 brief and front-loaded with the core purpose, followed by cost. It is slightly redundant due to bilingual text (English and Korean) but remains efficient and easy to scan.

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?

With no output schema, the description fails to explain what the tool returns (e.g., validity boolean, formatted number, carrier, line type). An agent cannot predict the result structure, which is a significant gap for a tool that performs multiple checks. The cost note does not compensate for missing output details.

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 schema description for 'number' is complete with examples (01012341234, +country code format), so the parameter is well-documented. The description adds no extra semantics beyond the schema, matching the baseline for high 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 clearly states the tool validates and formats a phone number, checks carrier and line type, using specific verbs and a concrete resource. It is distinct from siblings like check_email_valid (email) and check_spam_number (spam), making its purpose unambiguous.

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 usage for phone number validation but does not explicitly compare with alternatives or state when not to use it. Given the many sibling tools, explicit routing (e.g., 'use for phone validity, not email') would help, but the tool's name and function make the intended context inferable.

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