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

Annotations already declare this is a non-read-only, non-idempotent, non-destructive write. The description adds genuinely new behavior: each mailbox costs $3/month and the call returns a Stripe checkout link rather than provisioning immediately, which tells the agent payment is a required follow-up step. It doesn't say what happens to the mailbox before payment completes.

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?

Two compact sentences, front-loaded with the action and prerequisite, followed by price and return value. Nothing redundant.

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 2-parameter mutation with no output schema, the description covers cost and the checkout-link return. It omits what happens on payment abandonment, whether mailboxes are created inactive, and any limits on batch size, 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 coverage is only 50%, and the description clarifies one key constraint — the 'domain' value must be one purchased through this service. It adds nothing about the 'mailboxes' array (local part vs display name, batch limits, duplicates), leaving that entirely to the schema.

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 concrete verb+resource (add mailboxes to a domain) and scopes it to domains bought through this service, which separates it from add_contacts or connect_imap_mailbox. No sibling is named explicitly, so differentiation is inferred rather than stated.

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 phrase 'a domain you bought here' implies the prerequisite that the domain must already exist in the account, and the pricing note signals a billable action. But it never says when to prefer this over connect_zoho_mailbox / connect_imap_mailbox or what to do if the domain isn't owned.

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