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

Annotations already declare readOnlyHint=true, openWorldHint=false and destructiveHint=false, so safety is covered. The description adds genuine context beyond that: it discloses the return payload (message and unread counts) and a behavioral caveat that folder naming differs across Gmail, Mailcow and Outlook, which affects how results must be interpreted.

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 with the output contents front-loaded and the pre-move requirement immediately after. The provider-naming example is slightly idiosyncratic in wording but earns its place as a practical warning against fabricated names.

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 read-only, single-optional-parameter list tool with full schema coverage and explicit annotations, this covers what the agent needs: what comes back and why to call it. Return-format depth is unnecessary since no output schema exists but the counts are named.

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 optional 'mailbox' parameter is already documented as narrowing by mailbox label. The description adds nothing about the parameter, 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 identifies the resource precisely — 'the caller's mail folders with their message and unread counts' — which distinguishes it from siblings like mail_list_messages or mail_list_labels by naming the folder entity and the counts it returns. It lacks an explicit verb ('list'), but the noun phrase plus returned fields make the operation unambiguous.

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?

It gives a concrete when-to-use rule: 'Needed before any move,' with the rationale that invented folder names will not resolve. It does not name a specific alternative tool for related lookups, but the context is clear enough to steer invocation.

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