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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so safety is covered structurally. The description adds value beyond them: one query to the registry's public RDAP server, results cached up to 24h (a freshness caveat the agent would otherwise not know), and the registered=false edge case. It does not discuss failure modes such as unresolvable TLDs or rate limiting.

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?

Front-loaded with the verb and resource, then the returned fields, then the edge case, then the operational notes. Three sentences, no filler, every clause carries information an agent can act on.

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?

Low-complexity single-parameter lookup with an output schema that already documents the return shape, so the description need not explain return values. It nonetheless covers the semantics gap (registered=false), the freshness caveat, and the access path, leaving nothing an agent needs in order to call it 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 already documented there, including the URL/subdomain normalization rule. The description adds no syntax or format detail beyond the schema, 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 (a domain's RDAP registration record) and then enumerates exactly what comes back: created date, age_days, age_years, updated, expires, registrar, EPP status, nameservers, DNSSEC. No siblings exist, so there is nothing to disambiguate against, and the scope is unmistakable.

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?

Gives clear context for use and one important interpretation rule: 'registered is false when the registry has no record,' which tells the agent how to read a negative result rather than treating it as an error. It stops short of explicit when-not-to-use guidance (e.g. that it is domain-only, not IP/ASN), but with no sibling tools there is little alternative to route away from.

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