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

The annotations already declare readOnlyHint and openWorldHint, but the description adds useful behavior: it is a 'fast lookup', returns availability/renewal details/registrar link, and caps the input at 25 domains. This goes beyond what the annotations provide without contradicting them.

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 front-load the core purpose, then cover return contents, the pricing limitation, the sibling alternative, and the call limit. There is no filler or redundant restatement of the tool name.

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?

This is a simple single-parameter tool with a fully documented schema and no output schema. The description covers return values, constraints, and the key sibling routing, while annotations cover safety and open-world assumptions. Nothing significant 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 description coverage is 100%: the domains parameter is already fully documented with the format, examples, and the 25-domain limit. The tool description essentially restates this information rather than adding new parameter-level meaning.

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 the specific action 'check availability' against a specific resource: a list of full domains, with a concrete example. It also distinguishes itself from search_domain by noting that it does not provide registration quotes, so an agent can tell the tools apart.

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?

Explicitly says this is a fast lookup that does not supply registration quotes and directs the agent to search_domain for prices. It also implies via 'specific list' that this is for known domains, not discovery, though find_domains is not explicitly named as the discovery alternative.

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