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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), and the description goes well past them: it discloses that a non-indexed domain returns found=false but is still billed, that a bad-input 400 is free, that a data-source failure is not charged, and that payment is passed via _meta["x402/payment"] in USDC on Base. It also notes the exclusion of registrant personal data, which is exactly the kind of expectation an agent needs before calling.

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 sentence is front-loaded with the resource and return fields, then usage, then caveats — a sensible ordering. The price is stated twice ('$0.21 a call' and again as 'Price: $0.21 per successful call'), which is the one clearly redundant element in an otherwise dense, waste-free definition.

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-shape burden and does so by listing the exact fields (registration status, registrar, three dates, EPP codes, TLD, keywords, traffic, backlinks). Combined with input format rules and full billing outcomes for success, not-found, bad input and source failure, nothing an agent needs 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?

There is a single parameter and schema coverage is 100% — the schema already spells out 'registrable domain, no scheme or path, www. is dropped, no other subdomain.' The description's restatement of the www rule adds no meaning beyond the schema, so this sits at the baseline 3.

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+resource ('WHOIS overview of a domain') and enumerates the returned payload — registration status, registrar, dates, EPP codes, TLD plus keyword/traffic/backlink metrics. It is clearly distinguishable from narrow siblings like traffic_estimate or ranked_keywords because it declares itself a bundle. It stops short of naming the closest sibling (domain_overview), so an agent facing an overlap has to infer which one to pick.

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?

'Use when checking a domain's age, owner registrar or expiry' gives concrete triggering conditions, and the phrasing 'Send a registrable domain (www. is dropped)' tells the agent what input shape is acceptable. There are no explicit exclusions or named alternatives, which is the only thing separating this from a 5.

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