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

readOnlyHint=true already tells the agent this is a safe read, so the description's job is to add context — and it does: authoritative source, RDAP protocol, TLD coverage limits, and a per-call price of $0.005 USDC via x402, which is a cost signal no annotation conveys. It also documents the returned payload in place of a missing output schema. Rate limits, response encoding, and behavior for unregistered or unsupported domains are not stated.

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?

Purpose and source are front-loaded in the first clause, and the long field enumeration is justified because no output schema exists to carry that information. The price sentence is short and separately placed. The single 60-word sentence is dense but not padded; a semicolon or bullet structure would make the field list easier to scan.

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 one-parameter read tool with no output schema, the description covers the source of truth, the exact fields returned, coverage boundaries, and the monetary cost of the call — everything an agent needs to decide and invoke correctly. The absence of explicit sibling routing is the only meaningful gap, and that is scored under usage guidelines.

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 defines `domain` with an example, so baseline is 3. The description adds only indirect meaning — that the domain must sit under a gTLD or RDAP-publishing ccTLD — and says nothing about subdomains, IDNs, normalization, or what happens if a non-registrable hostname is passed.

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?

Names the exact resource (domain registration record) and its source (authoritative registry via RDAP, successor to WHOIS), then enumerates the returned fields so an agent knows precisely what question this tool answers. It does not explicitly differentiate from the nearby `dns` sibling, which an agent could reasonably confuse with registry lookup, so it stops short of 5.

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 explicit when-to-use/when-not-to-use routing or named alternative, so the agent must infer that this is the registry-registration lookup rather than a DNS resolver. The one piece of real guidance is the negative scope: 'Covers gTLDs and the ccTLDs that publish RDAP', which tells the agent when the tool will fail or return nothing.

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