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

Beyond the basic readOnlyHint=false annotation, the description discloses the mutation semantics, the exact seven-day extension, the precondition that the mailbox must be unexpired, and the key outcome that no new credential is returned. This is strong behavioral transparency for a zero-parameter tool.

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 sentences, both essential. The first is front-loaded with the action and duration, and the second conveys an important non-obvious outcome (no new credential). There is no filler or redundancy.

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 tool with zero parameters and an output schema, the description fully covers the operation, the precondition, and the expected credential behavior. Nothing an agent needs to call it correctly is missing.

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 tool has zero parameters and no schema properties to document, so the baseline of 4 applies. The description adds no parameter-specific meaning because none exists to explain.

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 specific verb and resource: 'renew your current unexpired mailbox for seven days.' It also distinguishes itself by explicitly noting that no new credential is returned, which sets it apart from related mailbox tools like close_mailbox or update_mailbox.

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 provides clear context: use this tool for a current, unexpired mailbox and to extend it by seven days. However, it does not explicitly name alternatives or state when not to use it, such as for expired mailboxes or other mailbox modifications, so it falls just short of explicit routing.

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