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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so safety is covered. The description adds real value beyond them by stating 'No user data', which tells the agent no auth or user scoping is needed, and by enumerating what the response covers (prices, storage, referral).

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?

A single compact sentence with the substantive content front-loaded and zero filler. The phrase 'how to earn by bringing others' is slightly awkward where 'referral program' would be clearer, costing a perfect score.

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 static, parameterless lookup with no output schema, the description covers the subject matter well enough to call it correctly. It does not spell out the shape of the returned plan list, but with no output schema that is a minor gap rather than a blocker.

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 takes zero parameters, so the baseline is 4 and there is nothing for the description to disambiguate. No parameter-level detail is needed or missing.

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 names a specific resource with concrete content: plan prices, what each plan stores permanently, and the referral earning program. That clearly separates it from siblings like buy_plan (purchase) and account_status (user state). It lacks an explicit verb (e.g., 'Returns' or 'Lists'), which keeps it just short of a 5.

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 only implied: an agent can infer this is the reference tool to consult before calling buy_plan, but the description never says so or names buy_plan as the next step. There are no explicit when-not conditions either, leaving routing to the sibling set to inference.

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