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

Annotations already declare the mutation profile (readOnlyHint=false, destructiveHint=true, idempotentHint=true), so the description doesn't need to restate that. Instead it adds non-structured context the agent can't get elsewhere: a plan gate, per-plan quota caps, and the fact that DNS records are returned and must be configured before verification. It does not explain what 'destructive' means here (quota consumption, whether an unverified domain can be removed), which keeps it out of 5 territory.

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?

Four short sentences, front-loaded with the action and the return behavior before the quota details. Minor redundancy between 'Builder accounts get 5' and 'Requires Pro or higher,' but nothing is wasted or buried.

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 one simple parameter, no output schema, and rich annotations, the description covers the essentials: what it does, what it returns, plan gating, and the verification prerequisite. Remaining gaps — error behavior on duplicate domains and reversibility — are modest for a single-parameter registration tool.

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?

There is a single parameter with 100% schema description coverage, including a hostname example, so the schema already carries parameter meaning. The description adds nothing further about the 'domain' argument (format constraints, subdomain rules, whether it must be unique). Baseline 3 applies when the schema does the work.

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 opens with a specific verb+resource: 'Register a new custom domain.' That cleanly separates it from list_domains, update_domain, verify_domain, and delete_domain in the sibling set. It stops short of naming a sibling explicitly, so it lands at 4 rather than 5.

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?

It supplies a real prerequisite ('Requires Pro or higher') and implies the follow-up workflow by saying the returned DNS records 'must be added before verifying' — pointing the agent toward verify_domain. It never states when *not* to use it (e.g. run check_domain_availability first, or use update_domain for an existing entry), so no exclusions are covered.

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