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

Annotations already cover safety traits (read-only, idempotent, non-destructive, closed-world). The description adds useful behavioral context beyond them: automatic renewal from balance on renewsAt, and the meaning of pending status as a placeholder that receives nothing until active.

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 front-loaded with the core purpose, then adds renewal and status details in compact sentences. It is appropriately sized for a simple list tool, with only minor awkwardness in the pending-status explanation.

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?

With zero parameters, read-only annotations, and an output schema present, the description needs mainly to explain what the list contains and important status nuances. It does that well by covering rented numbers, renewal behavior, and pending placeholders, though it could more explicitly mention active numbers.

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, so there are no parameter semantics to clarify. The schema is empty and fully described by its absence of inputs, making the baseline score appropriate.

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 identifies the resource and scope: phone numbers currently rented on this account. It is specific enough to distinguish listing rented numbers from sibling actions like buy_number and delete_number, though it does not explicitly name those alternatives.

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?

Usage is implied: an agent would use this to inspect currently rented numbers on the account. However, there is no explicit when-to-use guidance, no mention of when not to use it, and no direct routing to alternatives such as get_balance or buy_number.

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