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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is established. The description adds a small amount of context by noting authentication is required and previewing the returned fields, but it does not describe pagination, ordering, or any edge-case behavior.

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 description is a single efficient sentence with no filler. The main value statement and returned fields are front-loaded, and the redundant 'listMailboxes:' prefix is trivial.

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 simple read-only list operation with one optional parameter and no output schema, the description covers the essential information: the resource, the returned fields, and the auth requirement. It does not describe output structure, but that burden is not significant here given the explicit field list.

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%, so the only parameter, sessionKey, is already well documented in the input schema. The description adds no additional parameter-level semantics beyond the general authentication requirement.

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 uses a specific verb 'List' with a clear resource ('your mailboxes') and enumerates the exact fields returned: plan, payment term, status, and next renewal date. This clearly distinguishes it from sibling tools like createMailbox, deleteMailbox, and listMailPlans.

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 purpose itself implies when to use the tool: whenever the agent needs to enumerate the user's mailboxes. However, it does not explicitly state when this tool is preferable to alternatives like listMailPlans or getMailboxQuote, nor does it mention any exclusions or preconditions beyond authentication.

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