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

Annotations already declare non-read-only, non-destructive, non-idempotent, and closed-world. The description goes beyond them by disclosing the post-condition (site is served there once records are in place), the automatic HTTPS provisioning, and the plan entitlement — real behavioral context. It stops short of stating what happens on a duplicate/conflicting domain or whether the call is reversible.

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 and followed by the outcome and the plan caveat. Every sentence carries information: what it does, what you must do next, what happens after, and who is eligible. Slightly more than strictly needed but nothing is wasted.

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 correctly explains the return (DNS records) and the eventual outcome, plus entitlement gating. What is missing is failure/edge behavior — e.g., an already-claimed domain or a pending verification state — which an agent would want before calling.

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 description coverage is 100% for both required parameters, so the schema already documents 'site' and 'domain' formats. The description contributes only the constraint that the domain must be owner-controlled and repeats an example format already present in the schema. Baseline 3 is appropriate.

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?

States a specific verb and resource ('Adds a domain ... to a site') and immediately clarifies scope: an owner-controlled custom domain, distinct from list_domains or check_domain. The return value (DNS records to configure) is named, so the agent knows exactly what this call yields.

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?

There is an implicit eligibility rule ('Custom domains come with the Personal and Pro plans') which is genuinely useful gating context. However, it never says when to reach for this versus the closely related check_domain sibling, nor what prerequisite state the site must be in. Usage is implied rather than directed.

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