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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds information the annotations do not: a per-call cost of 1 credit, plus the shape of the result payload (registrar, dates, status, nameservers) in the absence of an output schema. It doesn't cover behavior for unregistered or invalid domains, which keeps it from a 5.

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?

One front-loaded sentence plus a parenthetical cost note, with zero filler. The returned-field list and the credit cost are the two highest-value facts and both appear immediately.

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 single-parameter, read-only lookup with rich annotations, the description covers purpose, output fields and cost. The only real gap is edge-case behavior (nonexistent domains, malformed input) and rate/credit limits beyond the per-call price.

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%, but the schema text ('string, required') is uninformative, so the description carries the real semantic load by establishing that the parameter is a domain name looked up via RDAP. It adds no format guidance (TLD handling, IDN, subdomain behavior), so baseline 3 is appropriate.

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 names a specific resource and protocol ('Domain registration data via RDAP') and enumerates the fields returned (registrar, dates, status, nameservers), so an agent knows exactly what this produces. It doesn't explicitly contrast itself with the nearest sibling, ssl_check, but the resource is distinct enough that confusion is unlikely.

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?

Usage is implied rather than stated: an agent can infer it should call this to get WHOIS-style registration facts for a domain. There is no explicit when-to-use guidance, no mention of when not to use it, and no pointer to ssl_check or other siblings for related domain queries.

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