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

Annotations already flag destructiveHint=true and idempotentHint=true, and the description adds genuinely valuable operational context: the 5-120 minute window, the no-SMS precondition, and what happens on failure (the rental runs to its end date rather than being cancelled). It does not say where the refund lands or how quickly, which is the one behavioral detail still missing for a money-moving destructive call.

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, the action and refund stated first, then the constraints. Every clause is load-bearing — the time window, the no-SMS rule, and the expiry fallback each change agent behavior.

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 destructive tool with no output schema, this covers the decision-relevant facts well: what it does, when it is legal, and what occurs otherwise. The remaining gap is post-call expectations — refund destination and timing — which matters for a tool whose whole purpose is a refund.

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 coverage is 100% and the single parameter already documents the accepted number formats in detail, including ignored characters. The description adds only the phrase "rented number," which carries no formatting or sourcing information beyond the 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?

States a specific verb and resource ("Cancel a rented number") plus the immediate consequence ("refund it"), which cleanly separates it from rent_number and from cancel_order, which operates on orders rather than rentals. An agent can pick this without opening the schema.

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?

Gives concrete eligibility windows (5-120 minutes after renting) and a hard exclusion (only if no SMS has been received), so the when-to-use condition is explicit. It stops short of naming a sibling as the alternative for the out-of-window case, leaving the agent to infer that an expired rental simply runs its course.

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