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

Annotations already declare the safe read-only/idempotent profile, so the bar is lower, and the description still adds real behavioral detail: the RDAP-vs-WHOIS backend distinction for .ru/.su/.рф, input normalization (subdomains reduced to the registered domain), and the fact that EPP status codes are returned with explanations. It does not mention rate limits or error behavior, but that is a minor gap against rich annotation coverage and an existing output schema.

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?

Purpose and routing are front-loaded, and every sentence carries information (returned fields, when-to-use, normalization, sibling pointer). The mid-section enumerating EPP status codes and DNSSEC is dense and slightly list-heavy for prose, which keeps it just short of ideal.

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 an output schema present and full annotation coverage, the description only needs to supply scope, routing, and backend quirks — all of which it does. Nothing an agent needs to select or invoke this single-parameter lookup is missing.

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% with a single required 'domain' param, so the baseline would be 3. The description goes beyond the schema by explaining that subdomains are accepted and reduced to the registered domain, and it hints at internationalized domains via the registry variants noted.

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 ('Look up a domain's registration in the registry') and enumerates precisely what it returns (expiry/days left, registrar, creation date, EPP status codes, nameservers, DNSSEC). It also explicitly separates itself from the sibling check_dns ('For DNS records use check_dns'), so an agent can route without opening either 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?

Gives two concrete trigger scenarios ('when a domain must be renewed or why it suddenly stopped resolving') and names the alternative tool for the adjacent use case (check_dns). When-to-use and when-to-use-something-else are both explicit.

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