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

Annotations cover the safety profile (readOnlyHint=true, openWorldHint=true, destructiveHint=false), and the description goes well beyond them by disclosing the per-call price ($0.0045 USDC), the payment rails (Base, Solana, Polygon, Arbitrum), the in-call x402 settlement, auto-paying clients, and a free sample path. Cost and payment mechanics are exactly the behavioral facts an agent cannot get from annotations and must know before invoking.

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?

It is front-loaded with purpose and return shape, then pricing and payment details. Nearly every sentence earns its place, though the return-field enumeration and the payment-chain list are dense and could be trimmed slightly without losing meaning.

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 lookup with no output schema, the description compensates by enumerating the returned fields and by covering the non-obvious cost/payment dimension. Nothing an agent needs in order to call this correctly and predict the response 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?

Schema description coverage is 100% and the single 'domain' parameter is fully documented in the schema, including the 'include TLD, exclude https://' guidance. The description repeats the example input but adds no format, normalization, or edge-case semantics beyond what the schema already provides, so the baseline 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?

The description gives a specific verb and resource ('Domain registration lookup via RDAP'), clarifies the mechanism ('modern WHOIS'), and enumerates the exact return fields (registrar, created_at, expires_at, name_servers, etc.). This implicitly but clearly separates it from sibling tools like dns_lookup (DNS records), ssl_check (certificates), and domain_check.

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?

Usage is only implied: it tells the agent how to make a free sample call and notes that x402 MCP clients pay automatically, but never states when to choose this over adjacent tools such as domain_check, domain_compliance, or dns_lookup, nor any exclusion conditions. The agent can infer the context but must do the routing work itself.

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