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

Annotations declare readOnlyHint=true, and the description reinforces that with 'Does NOT create or buy anything'. It adds behavior the annotations cannot convey: the free-first-mailbox pricing rule, yearly fee for additional mailboxes, that payment is made by the person, and that externally registered domains require DNS verification on the returned page.

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?

Front-loads the purpose, then the non-creating caveat, then pricing and DNS details. Dense with nested parentheticals, but every clause carries useful information; nothing is filler.

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 covers what is returned (price and a link) and the verification path for external domains. It omits error/invalid-address behavior and whether repeated calls are idempotent, which are minor gaps for a two-parameter read-like 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?

Schema coverage is 100%, so the baseline is 3; the description adds a real constraint beyond the schema by requiring the domain to be one of the person's domains, and its domain/localPart examples mirror the schema's address split.

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 ('Prepare a new mailbox') scoped to 'one of the person's domains', and immediately carves out what it does not do (create/buy). An agent can distinguish it from list_mailboxes or start_registration 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?

Explains the flow clearly: check the address, return the price and a link, the person accepts terms and creates it. Implies human-in-the-loop usage but does not explicitly name sibling alternatives (e.g. list_mailboxes, start_registration) or state when not to call it.

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