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

Annotations already cover readOnly/idempotent/openWorld, and the description goes well beyond them: it discloses the three-valued answer model, the failure modes (no RDAP server, timeout, 429), and that each answer carries a cheapest known USD price. Those are the behavioral traits an agent needs to reason about partial or degraded results, and they are stated explicitly.

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?

The core question is front-loaded and nearly every sentence carries information about inputs, answer semantics, or caveats. It is a dense, slightly run-on block, and the closing 'Served by NicheDB' attribution line is boilerplate rather than task-relevant, which keeps it from a 5.

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 takes on the burden of explaining returns and does so fully: the three outcome values, what registered entries include (registrar, expiry), what unknown implies, and the price field. For a two-parameter read-only lookup, nothing an agent needs to call or interpret the tool is missing.

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% and the parameter descriptions already spell out the three input forms (full name, bare word plus tlds, comma-separated list). The description restates essentially the same forms without adding new syntax, constraints, or limits, so it sits at the baseline where 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 leads with a concrete question ('Is a domain name registered?') and names the mechanism (RDAP, querying each registry), which tells an agent exactly what the tool does. It does not, however, differentiate itself from sibling domain-checking tools such as profullstack_check_domain_availability or profullstack_bulk_domain_check, leaving the agent to infer the boundary.

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 strong guidance on how to interpret answers ('not_registered ... is not a promise that it is for sale', 'unknown ... is never a yes'), which is genuinely useful usage context. But it never says when to prefer this tool over a sibling domain checker or when not to call it, so the alternative-selection guidance is only implied.

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