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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), so the bar is lower, and the description still adds real behavioral context: an RDAP-first strategy with IANA bootstrap and 22 fallback servers, port-43 WHOIS for 60+ TLDs, and a `_source` field identifying which backend answered. It does not mention rate limits or failure modes when all backends miss.

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 sentences, all front-loaded: what it returns, how it resolves it, and when to use it. No filler or repetition of the schema.

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 values need not be explained, and the description covers backend behavior and the `_source` discriminator well. The only omission is acceptable-input format for `domain`, which an agent calling this cold would benefit from.

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 one parameter with 0% schema description coverage, and the description never states the expected domain format (bare registrable domain vs. URL vs. subdomain, IDN handling). The parameter name is self-explanatory, so this is a minor but genuine gap rather than a severe one.

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 gives a specific verb+resource (registration data for a domain) and enumerates the exact fields returned (registrar, created/updated/expiry, nameservers, status), so an agent knows what it gets. It does not explicitly differentiate from the sibling domain_lookup, which is the only real gap.

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 states a clear use case: 'Use it for domain age, expiry or registrar checks.' That is a concrete selection cue. There is no explicit when-not or naming of the overlapping siblings (domain_lookup, dns_records), so it stops short of a 5.

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