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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, fully covering the safety profile. The description adds no extra behavioral traits such as error behavior, permissions, or rate limits, so it provides no context beyond the annotations—worth a 3 given the lower bar set by rich 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, front-loaded sentence that states the operation and target with zero filler. Every word earns its place; this is appropriately concise for a simple get-by-ID tool.

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 operation with an output schema present and annotations covering idempotency and non-destructiveness, the description is fully sufficient. An agent has everything needed to invoke it correctly; the sibling list also disambiguates it from related mailboxes tools.

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 mailboxId already described as 'Mailbox ID'. The description merely repeats the idea of fetching by ID without adding format, example, or relationship details, so it does not meaningfully raise understanding beyond the schema.

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?

Description uses a specific verb ('Fetch') and resource ('a single mailbox'), and the qualifier 'by ID' plus 'single' clearly distinguishes it from mailboxes.list and mailboxes.create/delete. An agent can immediately tell this is a targeted read operation without opening the sibling schemas.

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 description clearly implies this tool is for retrieving one specific mailbox when an ID is available, which provides adequate usage context. It does not explicitly name alternatives like mailboxes.list, but the 'single' vs. implied multiple distinction is strong enough that no exclusion is strictly necessary.

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