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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safe-read nature is covered. The description adds useful context beyond annotations: the account-level scope ('registered on the account') and the inclusion of DNS verification records in the result, which helps the agent know what to expect. This is modest but real added behavioral context.

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 one sentence, front-loaded with the action and resource, and every word earns its place. It is appropriately sized for a simple list operation and includes the key qualifier about DNS records without any fluff.

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 0-parameter, read-only list operation with an output schema present, the description completely covers the essential context: what is listed, the account scope, and the notable included detail (DNS verification records). Nothing an agent needs 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?

The tool accepts no parameters, so there are no parameter semantics to add. With schema description coverage at 100% and an empty parameters object, the description's job regarding parameters is trivially satisfied; the baseline of 4 for a zero-parameter tool 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?

The description uses a specific verb ('List') and resource ('custom domains registered on the account'), and adds a meaningful qualifier about DNS verification records. It clearly distinguishes itself from sibling tools like add_domain and remove_domain by focusing on read-only enumeration.

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?

The description implies its use whenever an agent needs to see the account's custom domains and their DNS records, but it provides no explicit guidance on when to choose this over alternatives or when not to use it. For a 0-parameter list tool with no competing list-domain sibling, this implied usage is acceptable but not explicit.

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