List email domains for a site

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

With no annotations provided, the description carries the behavioral burden. It discloses the API key requirement and explicitly states 'Does not spend,' which is valuable for an agent deciding whether the call is safe. It does not mention response format or error/rate-limit behavior, but for a read-only listing tool this is a reasonable disclosure.

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, each with a distinct purpose: what the tool does, what is required, and where it fits in the workflow. No filler or repetition of schema 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?

For a one-parameter list operation, the description covers purpose, prerequisites, workflow placement, and the non-spending behavior. It does not describe the exact output shape, but it does mention the key output contents (prices and stock), making it sufficiently complete for an agent to invoke the tool.

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 input schema already fully documents the single parameter ('site') with an example and pattern. The description adds no new parameter-level detail, so the baseline of 3 applies since schema coverage is 100%.

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 states a specific verb ('List'), a precise resource ('available email provider domains'), and the relevant details (prices, stock, target site). It clearly distinguishes this from other email_* operations and identifies the domain-picking purpose.

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?

It explicitly says to use this before email_request and notes that it requires an API key, giving clear usage context. It does not explicitly call out when not to use it relative to the many sibling email tools, but the main intended flow is clear.

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