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

With no annotations, the description carries the burden well: it discloses the return payload (ACM DNS validation CNAMEs for apex and www) and the two operational prerequisites that cause failure. It does not cover permissions, idempotency, or conflict behavior if the hostname is already attached, which keeps it below a 5.

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, front-loaded with the action, then the return value, then the prerequisites. No filler and nothing restated from the schema or annotations.

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 two-parameter tool with no output schema and no annotations, the description covers action, side effects, return shape, and gating preconditions. The remaining gap is post-call guidance (DNS propagation, verification via get_custom_domain_status), which an agent would likely want next.

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 50% – hostname is documented as the apex hostname, but site_id is not. The description adds meaningful context by implying hostname is apex-only and that www is generated, and by tying site_id to a live prod deployment, but it never defines site_id's format or how to obtain it.

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 names a specific verb ('Attach'), a specific resource ('externally owned domain'), and the exact target ('a site's live prod deployment'). The 'externally owned' qualifier also cleanly separates it from sibling tools like disconnect_custom_domain and get_custom_domain_status without needing to name them.

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?

Preconditions are spelled out concretely: an active prod deploy and an active domain subscription, with an explicit instruction to subscribe in the console first. It stops short of naming the follow-up tool (get_custom_domain_status) or stating when not to call it, so it is clear but not fully routing.

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