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

Goes well beyond the readOnly/openWorld/idempotent annotations: it discloses the data source (RDAP with rdap.org bootstrap), the availability heuristic (no RDAP record + no DNS), the young-domain flagging rule (<30 days), pricing ($0.002/call, 10 free/day, x402 payment-required result), and error behavior (isError on invalid input/upstream failure, not charged). This is unusually rich behavioral context.

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?

Dense but front-loaded: the return payload leads, followed by source, heuristic, usage, pricing, and error handling. Every sentence carries distinct information with no filler.

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?

Despite an output schema existing, the description fully covers what an agent needs: the input format, the availability semantics, the freshness flagging, cost, and error handling. Nothing required to call 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?

With a single parameter at 100% schema description coverage, the schema already documents the required 'domain' input including format guidance. The description adds no format or syntax detail beyond the schema, so the baseline of 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+resource ('Who registered a domain') and enumerates the returned fields (registrar, creation/expiry dates, status locks, nameservers, availability). It explicitly differentiates itself from dns_lookup, so an agent can distinguish it from siblings without opening the 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?

Contains an explicit 'When to use' clause scoped to registration and expiry, and names the alternative ('for DNS records use dns_lookup'). The condition that selects the sibling is spelled out, leaving nothing to inference.

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