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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered structurally. The description adds the result ordering (newest first) and an explicit prompt-injection warning about untrusted sender content, which is genuine added context, though the warning is aimed at message-reading paths rather than at a mailbox listing and no pagination or scope-default behavior is described.

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?

The useful scope statement is front-loaded, but roughly two-thirds of the text is a security caveat about email content that is only loosely tied to listing mailboxes. That space could have carried pagination or filtering behavior instead.

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 an output schema present and full annotation coverage, the description need not explain return values or safety. The ordering hint plus the injection warning are enough for a simple read-only list tool, though the default tenant scope when omitted is still unstated.

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%, with limit, cursor, and tenant each documented including the external-id-versus-Groundmail-id choice for tenant. The description adds nothing beyond the schema, so the baseline of 3 applies.

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 clear verb and resource ("List mailboxes") plus an ordering guarantee ("newest first"). It does not differentiate this from nearby siblings such as list_mailbox_threads or search_mailbox_messages, so an agent must infer the boundary from the names alone.

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?

There is no guidance on when to use this versus list_mailbox_threads, search_mailbox_messages, or list_tenants, and no mention of how the optional tenant filter should be chosen. The second sentence is a safety warning, not usage direction, so callers get no routing help.

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