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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful behavior beyond that: it tells the agent the call is scoped to a single mailbox and what each result carries (exact name, message count, unread count).

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 sentences, no filler. The scope and contents are front-loaded and the interoperability hint follows as the payoff sentence.

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?

No output schema exists, and the description compensates by naming the returned fields. For a one-parameter read-only list tool whose annotations carry the safety profile, nothing needed to invoke it correctly is missing.

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% and the single 'mailbox' parameter is documented as a full mailbox address. The description's 'in one mailbox' phrasing is consistent with the schema but adds no format or syntax detail, so the schema carries the load — baseline 3.

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?

Specific verb + resource + scope: 'List every folder in one mailbox', with the folder types enumerated (Inbox, Sent, Drafts, Trash, Junk, custom). The 'one mailbox' scoping implicitly separates it from list_mailboxes and list_messages, but no sibling is named explicitly, so an agent must infer the boundary.

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?

Gives clear context for why to call it: 'The names it returns are the valid values for the folder and destination parameters of the other mail tools' — effectively a prerequisite-discovery instruction for move_message/label_message and friends. It lacks an explicit when-not or a named alternative, so it stops short of a 5.

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