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

Annotations already mark this as read-only, idempotent, and non-destructive Mendable. The description adds valuable behavioral detail: no authentication requiredks, and it explains the output wrapping workaround (backend sends a bare array that FastMCP would reject as a top-level list). This goes well beyond the annotations and is consistent with them.

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 essential purpose and usage, then gives a compact, well-organized return format summary and a useful backend-wrapping note. Every sentence adds value; nothing is redundant.

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?

Given the tool has no parameters and annotations already declare its read-only, idempotent nature, the description covers everything an agent needs: what it returns, when to use it, and a noteworthy output-format quirk. The presence of an output schema also reduces the burden, but the description exceeds it.

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 parametersaren't needed, so the description does not need to add parameter meanings. Per the rubric, 0 params receives a baseline of 4, and the description appropriately focuses on output and usage instead of nonexistent parameters.

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: 'List available Maginary products/plans with pricing.' This clearly distinguishes the tool from siblings like checkout, get_balance, and create_account, and the use case is immediately obvious.

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 gives explicit context for when to call it: 'Use this to present purchase options to the user.' It also notes that no authentication is required and highlights a recommended product. It stops short of naming alternatives or exclusions, but the usage context is clear.

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