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

Annotations already declare readOnlyHint and closed-world scope, and the description adds real context beyond them: the returned fields, a per-mailbox computed capability ('whether an agent can read each one now'), and the mail:read permission prerequisite for actually reading mail. It stops short of stating ordering or whether mailboxes without read access behave differently, but the value add is substantial.

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 tight sentences: the resource and returned fields come first, and the permission caveat follows as supporting context. No filler or redundancy.

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 and no parameters, the description carries the burden of describing the response, which it does by listing the fields and the readability flag. Annotations cover the safety profile, so nothing essential is missing for a correct invocation.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The description correctly implies an unfiltered, account-scoped listing.

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 ('List the email mailboxes of this account') and enumerates what is returned (address, status, plan, expiry), which cleanly separates it from siblings like list_messages, list_domains, or create_mailbox. An agent can identify the target resource 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?

Usage is only implied: check mailboxes and per-mailbox readability before reading mail. It names no alternative tool and gives no explicit when-not-to-use guidance, so the agent must infer the routing from the resource name alone.

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