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

Annotations already cover readOnly/idempotent/non-destructive, and the description adds genuinely useful behavior: auth requirement, plan limits (Starter 15, Pro 200, Free 0 with HTTP 402 plan_limit_exceeded), status enumeration, and DNS auto vs manual mode. The value is undercut by a large block describing DELETE /api/v1/domains/:id cleanup semantics, which belongs to a different operation and can mislead about this tool's scope.

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 purpose is front-loaded, but the body is a run-on paragraph mixing return-shape spec, plan quotas, MX/SPF/DKIM/DMARC publishing instructions, receiving-latency notes, inbox creation, and full delete/cleanup semantics. Much of this does not earn its place in a read-only list tool's description.

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?

With no output schema, the description does the necessary work of describing the returned object and its status/hint/warnings fields, and annotations carry the safety profile. It is complete enough to call correctly; the failure is excess and misdirected detail rather than missing essentials.

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?

Zero parameters, so per the rubric the baseline is 4. The description instead spends its budget documenting the return shape (data array with id, domain, status, status_detail, mail, records, warnings, dns), which is useful but is not parameter semantics.

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 opening sentence gives a specific verb and resource ('list the custom domains on the signed-in account') plus scope ('including DNS status'), and it clarifies it is read-only with no inputs. It also implicitly separates itself from add/verify siblings by stating 'Cooper does not register a domain or create a zone.' An agent can identify the tool without opening the schema.

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?

Explicit when-to-use: 'Use this when you need the custom domains on the signed-in account.' It also signals what the tool is not responsible for (no domain registration, no zone creation). However, no sibling tool is named as an alternative, and the long digression into inbox creation and domain deletion blurs the boundary rather than sharpening it.

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