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

Goes well beyond the annotations: it discloses that failed data sources are not charged, that the call costs $0.003 USDC via x402 on Base, that an unpaid call returns the payment requirement rather than an error, and that invalid input is free. These billing and failure-path semantics are exactly the kind of context annotations cannot convey and are critical for an agent deciding whether to spend.

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 dense sentences, front-loaded with the returned data, then cost, then edge-case billing. No filler and no repetition of the title or 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?

Although there is no output schema, the description enumerates the return payload fields, the price, the payment-required path, and the free-failure conditions. An agent has everything needed to decide and invoke correctly.

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 the single `domain` parameter is fully documented in the schema, including the email/URL normalization behavior. The description adds no parameter-level detail, so the 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?

States a specific verb (look up) and resource (domain age and registration via public RDAP) and enumerates exactly what is returned: registration/expiry/last-changed dates, age in days, registrar, status flags, nameservers. This clearly separates it from the sibling domain_trust_score, which implies a reputation judgment rather than raw registry data.

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?

The description establishes context (public RDAP data) and the billing model, which implicitly frames when the call is worthwhile, but it never states when to prefer this over domain_trust_score or when a lookup is unnecessary. Usage is implied by the returned-field list rather than explicitly guided.

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