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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavioral context beyond the annotations: a free-sample quota and a $0.002 USDC per-call cost via x402, which is important for an agent deciding whether to invoke it.

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 tight sentences with no filler, and the returned-data summary is front-loaded ahead of the pricing note. Slightly list-like rather than sentence-structured, but every clause carries information.

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 tool with an output schema, the description needn't explain return values, and it covers the data domain plus cost. The only real gap is the absence of sibling differentiation, which the output schema and annotations cannot compensate for.

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% for the single 'domain' parameter, so the schema already documents it. The description adds no additional semantics (e.g., whether URL vs bare domain affects behavior), which is the expected baseline when the schema does the heavy lifting.

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 lists the returned data fields (registrar, created, expiry, age, status codes, nameservers, DNSSEC) but never states a verb or explicitly says it returns registration facts for a given domain. It also does not distinguish itself from siblings like domains__availability or domains__dns, so the agent must infer the purpose from the title and field list.

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?

There is no when-to-use guidance, no prerequisites, and no reference to alternative tools. The pricing/quota sentence is operational context, not usage guidance, so the agent gets no help choosing between this and the many sibling domain tools.

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