Zipcar MCP Server

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

With no annotations provided, the description carries the full burden. It successfully discloses the availability validation constraint but omits other behavioral traits such as idempotency, failure modes when the car is unavailable, payment implications, or whether extensions can be partial vs. only end-time extensions.

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?

Optimal length at two sentences. The first sentence front-loads the core action, while the second provides essential constraint information without redundancy. No filler or tautology present.

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?

Adequate for a two-parameter mutation tool, covering the primary function and key business constraint. However, given zero annotations and no output schema, it lacks completeness on error states, side effects (billing), and success indicators that would help an agent handle the response.

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%, documenting both 'reservation_id' and 'new_end_time' (including ISO 8601 format). The tool description adds no parameter-specific details, but per rubric guidelines, high schema coverage establishes a baseline of 3.

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 specific action (extend) and resource (existing Zipcar reservation), distinguishing it from sibling 'reserve_car' by emphasizing 'existing.' The scope (to a later end time) is precise, though it could explicitly contrast with 'end_trip' for maximum clarity.

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?

Provides a critical usage constraint (car must be available for extended period), which implies when the tool will fail. However, it lacks explicit guidance on when to choose this over 'reserve_car' for overlapping times or whether extensions incur additional charges.

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