add_custom_domain

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

No annotations are present, so the description must carry the full burden. It states that the tool returns DNS instructions for manual configuration, but it fails to disclose crucial behaviors: whether it validates domain ownership, what happens if the domain or subdomain is already in use, and if the operation is idempotent. The description is insufficient for an agent to understand side effects.

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 a single, concise sentence (16 words) that is front-loaded with the action. It contains no unnecessary words and effectively conveys the core purpose and output.

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?

Given the tool has 3 required parameters and no output schema, the description adequately states the purpose and return value (DNS instructions). However, it lacks details about error conditions, prerequisites, and the exact mapping process, which makes it somewhat incomplete for an agent to fully understand the workflow.

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 coverage is 100%, so the baseline is 3. The description adds minimal extra semantics beyond the schema—it provides an example of the domain format but does not explain the role of project_id in more depth. The parameter descriptions in the schema are already clear.

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 ('Register') and resource ('custom domain'), clearly stating the action: mapping a custom domain to a Run402 subdomain. It also distinguishes from sibling tools like list_custom_domains or remove_custom_domain by describing the registration process and the output (DNS instructions).

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?

No guidance is provided on when to use this tool versus alternatives (e.g., check_domain_status, register_sender_domain). There is no mention of prerequisites, such as domain ownership verification, or cases where this tool should not be used.

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