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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so credit goes for the business rules added on top: monthly credits reset each cycle and do not roll over, pack credits never expire, and there is no cost or auth. It does not say anything about caching or ordering of results, which keeps it below 5.

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, front-loaded with what is listed, then the non-obvious credit-expiry rule, then the cost/auth fact. No filler and every clause carries information an agent would otherwise have to discover by calling.

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?

With no parameters and no output schema, the description carries the return-shape burden and does so by naming the three fields, and it clarifies the billing semantics that would otherwise confuse a follow-up subscribe or buy_credits call. Nothing needed to call this correctly is missing.

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. The parenthetical field list gives a hint of what comes back without pretending to document inputs.

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 ('List the monthly subscription plans') and enumerates the returned fields (name, credits per month, price). The clause about pack credits being separate implicitly separates it from list_credit_packs, though that sibling is never named.

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?

'Free; no key needed' removes a common prerequisite but never says when to call this instead of list_credit_packs, get_subscription_status, or subscribe. The distinction from credit packs is implied through the credit-expiry remark rather than stated as routing guidance.

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