Extend Rental

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

Annotations already show this is a mutating, non-idempotent operation. The description adds an important behavioral and financial disclosure: it charges the user's balance at the current catalog price, which is not visible in the schema.

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 concise sentences with no filler. The primary action is front-loaded and the billing consequence follows immediately, making the description easy to parse.

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 two-parameter mutating tool with no output schema, the description covers the action, the active-rental prerequisite, and the key financial side effect. It does not describe return values, but that is not essential for tool selection or invocation.

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 descriptions already cover both parameters (100% coverage), so the baseline is 3. The description adds meaning by clarifying that rental_id must reference an active rental and that duration_hours results in a balance charge at current pricing.

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?

Clearly states the action ('extend'), the target ('an active rental'), and the change ('additional duration'). The 'active' qualifier and the billing effect distinguish it from create/cancel/get/list rental siblings.

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?

Explains the precondition that the rental must be active, which tells an agent when this tool is applicable. It does not explicitly name alternatives or when-not-to-use conditions, but the context is clear among the rental sibling tools.

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