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

With no annotations, the description carries the full burden. It explains the return format (JSON keyed by WHOIS server), the fields included (Domain Name, registrar details, dates, etc.), the default behavior (registry server only), and the trade-off of include_registrar (slower but more complete). It also discloses cost. It does not mention error handling or rate limits, but for a read-only lookup this is a solid, transparent description.

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?

The description is four sentences, front-loaded with the primary purpose, followed by output structure, parameter guidance, and cost. Every sentence earns its place; there is no redundancy or fluff. It is appropriately sized for a simple two-parameter tool.

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?

The tool has a simple two-parameter schema, an output schema, and no annotations. The description explains the return format, the keying by server, the parsed fields, and the parameter behavior. It is complete for the tool's complexity. It could mention what happens on invalid domains or network errors, but the output schema and parameter descriptions already cover the essentials.

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 description coverage is 100%, so the baseline is 3. The input schema already fully describes both parameters (domain with example, include_registrar with explanation of true/false behavior). The description adds no new parameter-level meaning beyond what the schema provides, just restates the include_registrar behavior and adds cost/output context.

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?

The description opens with a specific verb+resource: 'Fetch WHOIS registration data for a domain.' It clearly identifies the tool's function and differentiates it from siblings like domains.rdap, domains.dns_records, and domains.lookup by focusing on WHOIS-specific data and the response structure keyed by WHOIS server host.

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 description gives clear guidance on the include_registrar parameter (when to use true vs false), but it does not explicitly state when to prefer this tool over siblings like domains.rdap or domains.lookup. The usage context is implied by the 'WHOIS' focus, but there is no direct 'use this instead of X' guidance, so it falls at the 'implied usage' level.

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