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

With no annotations, the description carries full behavioral burden and does so: it enumerates returned fields (registrar, dates, days to expiry, EPP codes, nameservers, flags, score, summary), discloses the payment model ($0.01 USDC on Base via x402, call once without x_payment to get the challenge), and specifies failure semantics (no-RDAP TLD is reported as failed, never guessed).

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 purpose and use case before diving into return values and payment mechanics; every clause carries information. It is dense and slightly long for a single tool, which keeps it just short of a 5.

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 read-only lookup with no output schema and no annotations, the description supplies everything an agent needs: return shape, limits, input normalization, failure behavior, and the two-step payment handshake. Nothing material 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 baseline is 3, but the description adds meaning the schema lacks: URLs and 'www.' prefixes are stripped, the 10-domain cap is enforced per call, and the x_payment retry flow is spelled out in prose. That is a modest but real increment over the structured fields.

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+resource ('look up domain registration over RDAP') and immediately scopes the job with example questions (expiry, registrar, registration status). It is clearly distinguishable from the SEO/health/page siblings, none of which touch DNS registration data.

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?

Gives concrete trigger conditions ('when someone asks when a domain expires, who its registrar is, or whether it is registered at all') and the batch ceiling of 10. It does not name a when-not case or an alternative tool, but there is no sibling that overlaps, so the guidance is effectively complete.

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