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

Annotations already establish this as a safe, closed-world read, so the bar is lower, and the description adds real value on top by disclosing the API-key scoping, that each hostname carries a status, and the operational meaning of 'active' (any address there reaches the inbox). It stops short of describing the return shape or whether any pagination is involved.

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?

Two short sentences with zero filler, front-loaded with what is listed, followed by the semantic note that makes the status field actionable. Every sentence earns its place.

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 zero-parameter read with annotations covering the safety profile, the description is nearly sufficient: it says what is returned conceptually (hostnames plus status) and what status means. The only remaining gap is the concrete response shape, which has no output schema to cover it.

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?

The tool takes no parameters, so the baseline is 4 and there is no parameter semantics the description could or should explain. Nothing here conflicts with the empty schema.

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 and resource ('List the hostnames') and clarifies that these are the receiving domains scoped to the current API key, which disambiguates the name 'list_domains' from a generic domain-management tool. It does not explicitly name a sibling, but the message/OTP siblings are clearly unrelated, so the purpose is unambiguous without that.

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?

Usage is only implied: an agent can infer this is the discovery call for which hostnames are available for inbound mail. There is no explicit 'use this when...' statement, no note on when to call it versus the message-reading siblings, and no stated prerequisites or exclusions.

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