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

No annotations are present, so the description carries the full behavioral burden. It does disclose cost ('1 credit per call'), which is genuinely useful non-schema context, but says nothing about read-only/non-destructive nature, authentication needs, behavior for unregistered domains, rate limits, or failure modes.

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?

Two compact sentences with zero filler; the core capability is front-loaded and the cost caveat follows in a short clause. Every word earns its place.

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 simple one-parameter lookup with no output schema, the description adequately enumerates the returned data fields and the credit cost. It is missing only edge-case behavior (unregistered domains, invalid TLDs) and input format guidance, which keeps it from a 5.

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 description coverage is 0% and the single parameter is undocumented in the schema beyond a regex pattern and length bounds. The description references 'a domain's' but never clarifies the expected format (bare registrable domain vs. hostname/subdomain) or whether IDN/internationalized input is accepted, so it only partially compensates.

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?

States a specific verb (look up) and resource (a domain's registrar, registration/expiry dates, nameservers) plus the data source (RDAP), which is enough to separate it from generic web or DNS tools. It stops short of naming the adjacent siblings it must not be confused with, such as dns_lookup, reverse_dns and ip_whois, so a 5 is not warranted.

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 only implied: the agent can infer it is for registration metadata, and 'via RDAP' scopes it to registry data. There is no explicit when-to-use or when-not, and with siblings like dns_lookup, ip_whois, reverse_dns and domain-adjacent site_audit tools in the list, the lack of routing guidance is a real gap.

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