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

Annotations already provide readOnly, idempotent, and openWorld hints. The description adds value by specifying the protocol (RDAP) and listing the exact data returned (registrar, registrant contact, dates, EPP codes, domain age). It does not contradict annotations and enriches the behavioral understanding.

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 sentences long, with the first sentence covering purpose and outputs, and the second providing usage guidance and sibling differentiation. Every sentence adds value, and no unnecessary words are present.

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 existence of an output schema, the description does not need to detail return structure. It effectively lists key return fields, explains when to use the tool, and contrasts with a sibling. Combined with annotations and schema, it provides a complete context for correct invocation.

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 coverage is 100%, so baseline is 3. The description does not elaborate on the parameters beyond what the schema already provides. It mentions the domain input implicitly but offers no additional meaning for format or force_refresh. Thus, no extra parameter semantics added.

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 'Fetch' and the resource 'domain registration data via RDAP', lists specific data fields returned, and explicitly distinguishes from the sibling tool 'check_ns' by contrasting its purpose. This provides a specific and differentiated purpose.

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 explicitly tells when to use the tool ('when asked who registered the domain, who the registrar is, or when the registration expires') and distinguishes it from a sibling ('distinct from check_ns which identifies the DNS nameserver provider'). This provides clear usage guidance.

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