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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered; the description adds genuinely useful operational context the annotations do not: a per-call price (2 credits, with the CAD-cent conversion) and the fact that schema-invalid input is rejected before billing. It stops short of failure semantics — what happens on an unregistered domain or an RDAP timeout, and whether those calls are billed.

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?

Front-loaded with the identity of the tool and its data, with pricing last where it belongs. Minor waste: the field list ('registrar, creation and expiry dates, days to expiry, status codes, nameservers and DNSSEC') is then restated almost verbatim in the return-shape sentence.

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?

With no output schema, the description supplies the exact return object shape itself ({ domain, registrar, created, updated, expires, days_to_expiry, status, nameservers, dnssec }), plus cost and validation behavior. For a one-parameter read tool, the only material gap left is error/no-data handling, which is marginal.

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% on the single `domain` parameter, including the format example and the refused private-suffix/IP-literal cases, so the schema carries the semantics. The description adds only the cross-cutting note that schema-mismatched input is rejected pre-billing, which is a billing rule rather than added parameter meaning. Baseline 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 names a concrete verb+resource ('Modern WHOIS over RDAP') and enumerates the exact registration data returned (registrar, creation/expiry dates, status codes, nameservers, DNSSEC). The WHOIS framing implicitly separates it from dns-lookup, but it never names that sibling or any other, so differentiation is left to inference.

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?

There is no statement of when to choose this over the obvious neighbors (dns-lookup for records, web-search/web-contents for page data), and no exclusions. The only conditional content is billing/validation behavior, which is not usage guidance.

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