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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so safety is covered. The description adds genuinely useful implementation context beyond that: resolution is via RDAP with a WHOIS fallback specifically for ccTLDs lacking an RDAP service, which tells the agent to expect degraded or variant behavior outside gTLDs. It does not mention failure modes if both sources fail, which keeps it below 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?

Two sentences: the first is a dense statement of outputs and resolution mechanism, the second is a one-line justification of why the tool matters. Nothing is redundant and the outputs are front-loaded.

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 one-parameter read-only lookup it covers what is returned, how it is resolved, and why it matters, which is sufficient to invoke it correctly. The only remaining gap is expected behavior when neither RDAP nor WHOIS yields data for an obscure ccTLD.

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 required parameter with 100% schema description coverage including an example ('acme.com'), so the schema fully documents the input. The description adds no format or normalization guidance (e.g., whether to pass a URL or subdomain), so the 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?

States a specific verb (get) and enumerates exactly what is returned: registrar, creation date, expiry date, days remaining. This clearly separates it from sibling checks like check_domain_reputation, check_ssl, or check_nameservers, which look at different attributes of the same domain.

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?

The closing sentence gives a rationale ('domain age is one of the strongest cold-email risk signals'), which implies the scenario — pre-send risk assessment on a cold-email domain. However, it never states when to prefer this over the many adjacent domain checks in the sibling list (reputation, blacklist, Google domain, tracking domain), so the routing decision is left to inference.

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