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

Annotations already establish that this is a read-only, idempotent, non-destructive operation. The description adds important behavioral context: the balance applies specifically to prepaid keys, while Free/Pro/Bot keys return a note pointing to quota headers. That is useful beyond what the annotations provide.

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 two compact sentences with no wasted wording. It front-loads the tool's scope and then adds the key-type exception, which is exactly the information an agent needs.

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 simple read-only balance lookup with no output schema, the description provides enough context to call the tool correctly. It explains applicability by key type and the fallback behavior for non-prepaid keys. Minor missing details, such as the exact response shape, are not critical here.

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 schema fully documents the single api_key parameter, including that it can be omitted when sent as X-API-Key. The description adds meaningful semantics by saying the key must be a prepaid key bought with buy_credits. This narrows parameter selection beyond the schema's format example.

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 precise resource: remaining prepaid credits on a key bought with buy_credits. It also distinguishes this from free, Pro, and Bot keys, which are metered by quota instead. This lets an agent tell it apart from buy_credits and quota-related behavior without opening the schema.

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 clearly identifies when this tool applies: checking prepaid balance on a key purchased via buy_credits. It also explains that other key types are metered by daily quota and will return a note saying so, which prevents misuse. It does not explicitly name a sibling alternative for quota checks, but the key-type condition is clear.

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