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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint, but the description adds valuable behavioral context: it is rate-limited, requires no API key, uses RDAP with WHATIS fallback, and returns specific fields. This goes beyond what the annotations express and helps the agent anticipate operational constraints. No contradiction with annotations exists.

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 front-loaded with the core purpose, followed by usage guidance and constraints, making it easy to scan. However, the final sentence 'Returns registrar, key dates, nameservers, and status flags' largely repeats the data points already listed in the first sentence. This minor redundancy prevents a perfect score.

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?

For a tool with one required parameter, an output schema, and annotations covering its safety profile, the description is complete. It covers purpose, usage alternatives, authorization requirements, rate limiting, and output highlights. No important operational information is missing.

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 covers 100% of the single parameter's meaning, including the subdomain normalization detail. The description does not add additional parameter semantics beyond what the schema already provides. Per the rubric, a baseline of 3 is appropriate when schema coverage is high.

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 opens with a specific verb and resource: 'Retrieve domain registration data via RDAP (with WHOIS fallback)' and enumerates concrete outputs such as registrar, key dates, nameservers, and EPP status flags. It also highlights risk states like clientHold and pendingDelete, which makes the tool's purpose unambiguous. The availability of a WHOIS fallback further clarifies behavior and distinguishes it from a plain RDAP lookup.

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?

Explicit guidance is provided: 'Use this for ownership, lifecycle, and expiry questions about a registered domain.' It also names alternatives with conditions: use dns_lookup for live DNS records and reverse_dns/asn_lookup for IP-level ownership. This leaves no doubt about when to choose this tool over its siblings.

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