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

Annotations already establish that the tool is read-only, idempotent, and non-destructive, so the description only needs to add non-obvious behavior. It adds 'newest first' ordering, but does not disclose pagination behavior or how invalid mailbox IDs are handled. No contradiction with annotations.

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?

A single sentence that front-loads the operation and resource, and includes the important ordering detail. There is no filler or redundant restatement of the title.

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 one-parameter, read-only list with an output schema and safety annotations, the description is largely sufficient: it names the resource and ordering. It stops just short of full completeness by not mentioning whether the response includes all emails or paginates, but this is a minor gap given the simplicity of the 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?

The input schema covers mailboxId completely with a 'Mailbox ID' description, so the schema is doing the heavy lifting. The tool description adds no parameter-specific meaning—'mailbox' is already evident from the parameter name and schema—so the baseline 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 clearly states a concrete operation—listing emails in a mailbox—and adds an ordering guarantee ('newest first'). It distinguishes mails.list from obvious siblings like mailboxes.list and mails.get, but it does not explicitly differentiate it from archived.list or state whether archived emails are included.

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?

The use case is implied: call this when you need the emails in a mailbox, sorted newest first. However, there is no explicit guidance about alternatives (e.g., mails.get for a single email, archived.list for archived emails) or exclusions such as whether drafts or archived items are included.

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