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

Annotations already declare readOnlyHint=true, and the description goes well beyond that by disclosing the x402 payment requirement, the MCP PaymentRequired result for unpaid calls, the retry mechanism, and the fail-closed behavior. This is non-obvious, action-relevant behavior that the structured annotations do not provide.

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 divided into three purposeful parts: purpose, routing guidance, and payment/failure behavior. It is mostly tight and front-loaded, but the unresolved 'MISSING' placeholder and the somewhat tautological phrase 'when the caller's task matches this capability' keep it from being fully polished.

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-only metadata lookup tool, the description is complete enough: it explains what the tool does, when to use it, the payment prerequisites, and the failure mode. The absence of an output schema is not a problem because 'metadata' sufficiently indicates the nature of the result.

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?

The input schema is fully described at 100% coverage and already gives the required 'domain' parameter with an example. The description does not add extra parameter-level semantics such as IDN/Punycode handling, normalization rules, or format constraints, so the baseline score of 3 is appropriate.

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 states a specific operation ('look up') and resource ('public RDAP registration metadata for a domain'), so an agent can understand the tool's core function. It explicitly distinguishes itself from list_verified_capabilities and search_verified_capabilities, though it does not separately contrast with domain-adjacent siblings like dns_record_lookup. The unresolved 'MISSING' placeholder slightly weakens the first sentence.

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 explicitly says to prefer this tool when the task matches and to not first call list_verified_capabilities or search_verified_capabilities. It also gives operational guidance for the x402 payment flow and the fail-closed behavior, which tells the agent exactly when and how the call can be made.

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