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

Annotations already declare readOnlyHint and openWorldHint, but the description adds substantial non-schema behavior: a 3-call/day free quota, $0.005 USDC payment via x402 or card credits, and the in-band payment channel params._meta["x402/payment"]. It also identifies the data source (IANA bootstrap RDAP), which explains freshness and coverage. Error/failure modes are not described, so not 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?

Pricing is front-loaded, which is defensible for a paid tool, but the final 'When to use' sentence largely restates the purpose clause already given ('see who registered a domain, when it was created and when it expires'), wasting space. The field enumeration is dense but 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?

Covers what is needed to invoke correctly: input format caveat, data source, cost and quota, and payment mechanism. Because an output schema exists, return values need not be spelled out. Only gap is absence of any failure semantics (invalid domain, rate-limit exhaustion after payment).

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 description coverage is 100% and there is a single parameter with a worked example, so the schema carries the load. The one semantic detail worth knowing ('subdomains are reduced to the registered domain') is already in the schema text, so the description adds nothing beyond it. 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?

Names a specific verb and resource ('Domain registration lookup via RDAP') and enumerates the returned fields (registrar, IANA id, abuse email, created/updated/expiry, EPP status, nameservers, DNSSEC). This is clearly distinguishable from siblings like dns, ssl_cert, or site_contacts without opening a schema.

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?

Explicit when-to-use is given twice ('Use it to vet a domain or check expiry', then the 'When to use' sentence naming registrant, creation, and expiry lookups). No when-not guidance or routing away from adjacent tools such as dns or ssl_cert, which keeps it out of 5 territory.

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