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 goes further by explaining that lookups hit the TLD registry directly, enumerating the possible status values, and warning that a lookup is not a reservation. It stops short of covering rate limits, auth requirements, or pagination behavior.

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 sentences, front-loaded with the core purpose, then the return values, then the caveats. Dense but each sentence carries information; the status enumeration is slightly long but genuinely useful since no output schema exists.

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?

With no output schema, the description correctly takes on the job of explaining return values (registered / available / likely_available / unknown, plus registrar, dates and prices). It also covers the batch limit and the reservation/premium caveat, so an agent has everything needed to call and interpret this tool.

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% for the single 'domains' parameter, so the schema already documents the array, its item format, and the 1–20 bounds. The description restates the 20-name cap and adds nothing about accepted input syntax beyond the schema's own examples, so 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 and resource ('Check whether domain names are registered') and even names the mechanism (RDAP, direct registry queries), which separates it from suggestion or appraisal tools. It never explicitly names a sibling such as check_name or search_domains_for_sale, so differentiation is inferred rather than stated.

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 gives the batch limit ('Up to 20 names per call') and a warning about what a lookup does not guarantee, which implies usage, but it never says when to prefer this tool over check_name, suggest_names, or search_domains_for_sale. No exclusions or alternatives are offered.

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