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

Annotations already declare readOnly, idempotent, and non-destructive nature. The description adds valuable context about the data (e.g., 60% are shells, parent_entity information) and the fields returned, without contradicting 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?

The description is concise, consisting of two sentences plus a related-tools line. It contains no fluff or redundancy, and efficiently conveys all necessary information.

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?

The description fully explains the tool's function, the returned data, and even a domain-specific nuance (parent entities for shells). It is complete for a simple lookup tool, though it doesn't elaborate on output format or pagination details, which are likely covered by the schema.

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 already provides thorough descriptions for both 'query' and 'page' parameters (e.g., 'Registrar name, alias, or IANA ID'). The description adds no new information beyond what schema provides, so baseline score of 3 applies.

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 verb 'look up' and the resource 'ICANN-accredited registrars', with specific search criteria (name, alias, IANA ID). It also lists the returned fields, making the tool's purpose unambiguous and distinct from sibling tools like whois or available.

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 when registrar information is needed, and mentions related tools (whois, available, expired) but does not explicitly state when to use this tool over those alternatives. However, the context is clear enough for an agent to select it appropriately.

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