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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds genuinely new behavioral context beyond them: lookups go over RDAP to the TLD's registry, and subdomains are normalized to their registered domain. It does not disclose failure modes (unregistered domain, TLD without RDAP service), which keeps it short of a 5.

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 sentences, zero filler, with the core action front-loaded and the input-normalization caveat second. Every clause earns its place.

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 single-parameter read-only lookup with no output schema, the description covers what the tool does, how the input is resolved, and where the data comes from. It stops short of describing what the result contains or how lookup failures surface, but nothing essential to invoking it correctly 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% and the schema already explains that URLs are accepted with only the host used. The description adds meaning beyond that by specifying the registered-domain normalization rule (www.example.co.uk becomes example.co.uk), which tells the agent how multi-label public suffixes are handled.

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 ('looks up when a domain registration expires') and names the resolution mechanism (RDAP at the registry for its TLD). This clearly separates it from the sibling check_certificate, which covers certificates rather than registration expiry.

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 purpose makes the use case self-evident, and the subdomain rule gives some input-preparation guidance, but there is no explicit when-to-use vs. when-not-to-use guidance and no reference to check_certificate as an alternative for a different question.

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