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

Annotations already declare readOnlyHint=true, openWorldHint=true, idempotentHint=false, destructiveHint=false, so the safety profile is covered. The description adds behavior the annotations cannot express: billing at $0.01/call, first 25 free, failed calls free, and the explicit warning that repeats are not free — which productively explains why idempotentHint is false. It stops short of describing error modes, rate limits, or freshness of RDAP vs WHOIS data.

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?

Return fields are front-loaded in the first sentence, usage second, example third, cost last — a sensible priority order with no filler. The pricing clause is dense ('x402 USDC or a card key (/credits), first 25 free; failed calls free') but every element is decision-relevant for an agent that must weigh call cost.

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 correctly compensates by naming the returned fields (registrar, dates, age, DNS, SPF/DMARC), and the read-only annotations cover safety. For a one-parameter lookup this is close to sufficient; only data-source caveats (RDAP vs WHOIS fallback, staleness) and failure semantics are absent.

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?

There is a single parameter at 100% schema description coverage ('Domain name, e.g. cloudflare.com'), so the schema already carries the semantics. The description's 'Example: name=cloudflare.com' merely re-demonstrates the same value without adding format constraints, subdomain behavior, or whether bare apexes vs hostnames are accepted. Baseline 3 is appropriate when the schema does the work.

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+resource ('WHOIS lookup for a domain') and enumerates the concrete payload: registrar, creation/expiry date, age, DNS records, and email auth (SPF, DMARC). That is far more than a restatement of the name. It does not, however, explicitly contrast itself with near-neighbors like domain_whois or domain_report, so an agent cannot rule those out from the text alone.

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?

It gives real use contexts ('lead research and phishing checks') and a worked example invocation, which is genuine when-to-use guidance. What is missing is any when-not or alternative routing: the sibling list contains domain_whois and dns_lookup, and the description offers no basis for choosing between them.

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