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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds authentication context and confirms no parameters are needed. It does not describe unauthenticated behavior or response details, but those are not critical for this simple read-only tool.

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 description is short and front-loaded with the core purpose. It includes useful operational notes about authentication and parameters. The only minor redundancy is the 'listUserDomains:' prefix that repeats 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?

For a simple read-only listing tool with annotations covering safety and a schema covering the only parameter, the description is adequate. It tells the agent what the tool returns conceptually (all domains owned by the user) and how to invoke it. It could name an alternative like listMailDomains or describe the response shape, but neither is essential.

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 provides 100% coverage for the optional sessionKey parameter, including its format and usage. The description's 'No parameters required' is accurate but already inferable from required: []. Thus the description adds little beyond the 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 states a specific verb and resource: 'List all domains owned by the authenticated user.' This clearly separates it from related domain tools like listMailDomains and other list-* siblings by scoping to domains owned by the user.

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 provides a clear prerequisite: 'Must be authenticated first.' It also clarifies that no parameters are required. It does not explicitly name alternative tools or when-not-to-use it, but the context is clear enough for a straightforward listing operation.

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