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

Beyond the annotations, it discloses the exact risk thresholds (fail under 30 days old or expired, warn under 180 days, expiring within 30 days or on hold), the 422 refusal behavior, and the full payment/auth model including that x402/MPP per-call payment is HTTP-only and that a missing key returns a payment requirement. That is precisely the operational context annotations cannot carry.

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 content is useful but poorly front-loaded (it opens with pricing) and materially redundant: '$0.005' appears twice and 'a registry without RDAP is refused free' is stated twice in near-identical form. Roughly a sentence of repetition could be cut without losing information.

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 paid, registry-backed lookup with an output schema already covering return values, the description supplies the missing pieces: cost, payment paths, refusal conditions, and risk thresholds. It restates the returned fields even though the output schema covers them, which is mild bloat rather than a gap.

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%, so the baseline is 3, but the description goes further on api_key by explaining that card checkout happens at /pricing and that per-call x402 payment is unavailable over MCP, which genuinely clarifies why the key parameter exists. The domain parameter's format guidance ('no URL, no IP') is left entirely to the schema.

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 states a specific purpose ('know which registrar holds a domain, how old it is, and whether it is a risky newly-registered domain') and enumerates the returned RDAP fields. It is clear about the resource and verb, but it never distinguishes itself from the sibling domain_check tool, so an agent has no basis for choosing between them from the description 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 a concrete when-not ('Not for registries without RDAP, e.g. .za/.co.za, .me, .de, .io, .eu: refused free (422)') plus the positive trigger ('Use when you need to know...'). However, it names no alternative tool, so the routing value stops short of the explicit 'use X instead' guidance that would earn a 5.

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