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

Annotations already cover read-only, idempotent, open-world, non-destructive behavior, so the bar is lower, and the description still adds meaningful context: the registration data source (the registry's RDAP server) and an explicit privacy limitation that no registrant names or contact details are returned. That limitation is real behavioral information an agent would otherwise only discover after calling.

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: a dense but front-loaded inventory of what is returned, followed by the scope limitation. Every clause earns its place, though the single long enumeration sentence is heavier than 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 no output schema, the description carries the burden of describing return values and does so thoroughly, field by field, plus the source of the data and the explicit exclusion of registrant details. The single fully-documented parameter leaves no ambiguity about how to call it.

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 the parameter accepts a domain, URL, or email address and that the registered domain is used. The description only says 'for one domain,' adding no syntax or format meaning 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?

A specific verb ('look up') plus resource ('domain') with an explicit enumeration of the returned data: registration status, registrar, key dates, status codes, nameservers, A/AAAA/MX/NS/TXT records, email provider, SPF/DMARC. It is clearly distinguishable from siblings like validate_vat, check_email, or search_company_registry.

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 use case is implied clearly by the enumerated outputs, but there is no explicit when-to-use/when-not guidance and no mention of alternatives such as check_email for email-focused validation or search_company_registry for registrant-level data. An agent can infer the fit, but the routing is left to it.

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