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

Annotations already declare read-only/idempotent/non-destructive, but the description adds material context the annotations cannot: the $0.01 USDC x402 cost, where the payment goes (_meta["x402/payment"]), and the billing rule that invalid input or upstream failure returns an error without charge. It also discloses the found=false outcome for TLDs lacking RDAP, which shapes how the agent must interpret results.

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?

Three tight sentences: what it returns, how it differs from check_cert, and payment/edge-case behavior. Each sentence is load-bearing and the primary purpose is front-loaded before the cost and edge-case details.

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?

With no output schema, the description carries the return-value burden and does so by naming the result fields plus the found=false sentinel. Payment mechanics and error/charging behavior round out everything an agent needs to call it correctly.

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% and the single 'domain' parameter already documents accepted forms (hostname or URL with example). The description adds no further syntax, normalization, or validation detail beyond the schema, 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?

Specifies the verb (check), the resource (domain registration expiry), and enumerates the returned fields — registrar, dates, days left, EPP status, nameservers — so the agent knows exactly what it gets. It also explicitly separates itself from check_cert, avoiding the most likely sibling confusion.

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?

States the use case (renewal risk on the registered domain), names the alternative for the adjacent case (check_cert for certificates), and covers the important negative case where a TLD publishes no RDAP service and found=false explains why. When-to-use, when-not, and the sibling alternative are all present.

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