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

Annotations already declare readOnly, idempotent and openWorld, so safety is covered. The description adds genuinely new behavioral context: the RDAP-first strategy (rdap.org, IANA bootstrap, 22 fallback servers), the port-43 WHOIS fallback for 60+ TLDs, and that `_source` reports which backend answered. This explains variability in results that annotations cannot convey.

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?

Three tight sentences with no filler; the payload (fields returned) is front-loaded, followed by the resolution mechanics and the usage trigger. Every clause 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?

An output schema exists, so return-shape explanation is unnecessary, and the description still flags the `_source` field and the fallback chain an agent needs to interpret results. The only gap is the accepted format of the `domain` input.

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 0% schema description coverage, so the description must carry the load and does not. It never says whether `domain` accepts bare registrable domains, subdomains, URLs, or IDN forms — a meaningful ambiguity for a WHOIS/RDAP tool.

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 and resource ('Registration data for a domain') and enumerates the returned fields (registrar, created, updated, expiry, nameservers, status), so the agent knows exactly what comes back. It does not explicitly distinguish itself from the close sibling domain_lookup, which is the only remaining ambiguity.

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?

Gives a clear use-case trigger: 'Use it for domain age, expiry or registrar checks.' That tells the agent when this tool is the right pick, but it names no alternatives or exclusions relative to domain_lookup or dns_records.

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