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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered; the description adds real behavioral context beyond that by naming the protocol (RDAP) and disclosing a genuine coverage limitation ('Not every country code TLD publishes RDAP'). It stops short of saying whether an unsupported TLD yields an error or an empty result, which is the one behavioral detail an agent would still need.

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?

Two tight sentences: the first front-loads what the tool does and what it returns, the second carries the only caveat worth knowing. Nothing is repeated from the title or annotations and there is no filler.

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 one-parameter, annotated, read-only lookup with no output schema, the description does the heavy lifting by enumerating return fields and flagging TLD coverage limits. The remaining gap is how failures surface (error vs. null data), which matters given the explicitly partial RDAP coverage.

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?

With a single parameter at 100% schema description coverage, the schema already documents the domain argument and even the normalization rule (URL or email reduced to its domain). The description adds no syntax, format or edge-case meaning beyond that, so the baseline of 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 ('looks up') and a specific resource ('a domain's registration data over RDAP'), then enumerates the concrete fields returned (registrar, creation, expiry, update dates, status, nameservers). This is clearly distinct from the DNS/IP-resolution siblings in the catalog, so an agent can tell what family of answer to expect without opening the 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?

Usage is implied rather than stated: an agent can infer this is the tool for registration/ownership/domain-age questions, but there is no explicit when-to-use framing and no named alternative (e.g. dns_lookup for records, hosting_check for hosting) to route against. The ccTLD caveat is a constraint, not guidance on choosing between tools.

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