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

The description adds substantial behavioral detail beyond the readOnlyHint annotation: it explains the null semantics of freeCreditsRenewAt, instructs the agent on what to do when the balance is too low, and explicitly says not to push a purchase. These are non-obvious behaviors that an agent needs to know.

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 compact and every sentence adds value: the first defines the return values, the second explains the low-balance handling policy, and the third provides relevant cost context. It front-loads the most important information and avoids fluff.

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 no-parameter, read-only tool with no output schema, the description is sufficiently complete. It explains the response semantics, the null case, the follow-up behavior with plansUrl, and the one-credit cost context, giving the agent everything needed to invoke and react to the tool correctly.

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 parameters and the schema is empty with 100% coverage, so there are no parameter details to document. This matches the rubric baseline of 4 for zero-parameter tools; no additional parameter compensation is needed.

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 clearly states a specific verb and resource: it returns the user's credit balance, premium status, and freeCreditsRenewAt. It is unambiguous and naturally distinguishes itself from sibling deck- and review-related tools, since no other sibling focuses on credit balances.

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?

There is no explicit statement of when to use this tool versus alternatives, nor any named sibling comparison. The last sentence, 'Each deck generation or expansion costs 1 credit,' implies this tool should be used in the context of checking affordability before generating or expanding decks, but the guidance remains implicit.

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