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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), and the description adds genuinely useful context beyond them: the authoritative data source (official IANA RDAP servers), the exact record types queried, and a rate limit plus paid fallback. It does not say how partial failures behave (e.g., DNS succeeds but port 443 has no certificate), which is the main remaining behavioral gap.

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, front-loaded with the payload contents before the access/pricing terms. The long parenthetical field list is dense but every item is a returned value an agent would want to know, so it earns its space; a semicolon-separated list would read slightly faster.

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 return-value burden and does so thoroughly for all three data planes. The notable gap is that the schema marks no required fields, yet 'domain' is obviously mandatory — the description implies it ('One call for a domain') without ever stating that domain must be supplied or what happens for an unresolvable domain.

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%: both parameters, including the tls default of true, are already documented in the schema. The description adds no syntax, normalization, or edge-case detail for 'domain' (subdomains, IDN, scheme prefixes) beyond the schema's own example, 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 concrete verb-plus-resource ('One call for a domain') and enumerates exactly what comes back: RDAP registration fields, live DNS record types, and TLS certificate attributes. None of the siblings (clinical_trials, federal_register, osv_vulns, prediction_markets, web_read) overlap, so an agent can route unambiguously.

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 by the domain-lookup framing rather than stated; there is no explicit 'use this instead of web_read when...' or when-not guidance. It does add a practical operating condition (25 free calls/day per IP, then $0.005 via x402), which helps an agent budget calls but is not selection guidance.

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