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

Annotations only say readOnly and open-world; the description adds real substance beyond that — it is a live registrar call with a stricter rate limit, it never reserves inventory, and pricing is quoted in EUR with VAT and is flat on renewal. The rate-limit and no-side-effect disclosures are exactly the kind of context annotations cannot carry.

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 short sentences, front-loaded with the core question and followed by the pricing qualifier, the no-side-effect boundary, and the rate-limit caveat. Every clause carries decision-relevant 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?

Single fully documented parameter, no output schema, and the description discloses both the return content (availability + first-year cost) and the operational constraint (rate limit). Nothing an agent needs in order to call 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% and the schema already lists accepted endings and diacritic handling, so the description doesn't need to repeat it. It adds only marginal parameter meaning (per-ending flat pricing), 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?

Specific verb+resource ('Check whether a domain name can be registered') plus the value it returns (first-year cost). It is clearly distinct from siblings like start_registration/start_renewal and list_domains, which operate on domains already owned or on purchases.

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?

'Does not reserve or buy anything' draws a firm when-not boundary and implicitly routes purchasing elsewhere, and the rate-limit note flags a precondition for use. It stops short of naming the alternative tool (start_registration) explicitly, as a 5 would.

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