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

The description adds meaningful behavioral context beyond the annotations: it discloses the double opt-in flow, that monitoring starts only after the confirmation link is clicked, and the types of monitoring performed. This is valuable since readOnlyHint=false already indicates a side-effecting operation but not its specific behavior.

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 sentences with no fluff: the first front-loads the core action and scope, the second explains the opt-in flow, and the third gives the critical usage restriction. Every sentence earns its place and the most decision-relevant information appears early.

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 subscription tool with no output schema, the description covers the purpose, the input semantics, the opt-in behavior, and the consent requirement. It does not describe the immediate return value or what happens on duplicate subscription, but these are secondary given the clear description of the side-effecting flow.

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%, so the baseline is 3. The description adds useful meaning by explaining what happens to the domains (certificate, registration, and security grade monitoring), the 'up to 5' constraint, and the consent requirement for the email parameter. This goes beyond the schema's simple field descriptions.

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 states a specific verb ('subscribe') and resource ('an email address ... to free daily expiry monitoring'), and clearly distinguishes this from the check_* siblings by emphasizing ongoing monitoring and email alerts. The parenthetical listing what is monitored further clarifies the tool's scope.

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?

The description clearly implies when to use this tool: when the user wants ongoing daily monitoring with alerts, as opposed to a one-time check. It includes a strong consent guideline ('Use only an address the user owns and explicitly asked to use'), though it does not explicitly name check_expiry or other alternatives as the contrast.

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