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

With no annotations, the description carries the full burden and does well: it discloses the 'No contact data' limitation, the payment model ($0.0045 USDC via x402 on Base or Solana), and that URLs are normalized. It leaves out rate limits and error behavior beyond what the schema already states for reserved names.

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?

Front-loads the use case, then segments Input, Output, and Price so an agent can scan the relevant part. Every sentence (including the contact-data exclusion and cost) contributes 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 lookup with no output schema, the description fully specifies the input, the return payload field-by-field, the cost and payment rails, and the 400/not-charged edge case. Nothing an agent needs to invoke it correctly 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 coverage is 100%, so the schema already documents the single parameter in detail; baseline would be 3. The description adds a concrete example ('e.g. example.com; URLs accepted') and a sample query string, giving marginal extra value.

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?

States a specific verb and resource ('check a domain') and enumerates the questions it answers: resolution, DNS/mail hosting, email authentication, age, expiry. No sibling tool overlaps, so an agent can select it unambiguously.

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 opening 'Use when an agent must check a domain' plus the enumerated questions gives clear context for invocation. It doesn't state exclusions or point to an alternative, but the sibling list contains nothing comparable, so no exclusions are strictly needed.

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