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

No annotations are provided, so the description carries the behavioral burden. It implies a read-only operation by saying 'Check' but does not state whether the operation is side-effect-free, what it returns, or whether there are rate limits. For a simple balance check, this is acceptable but not fully transparent.

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 terse and front-loaded with the core action. The second sentence adds practical usage guidance but repeats 'remaining,' making it slightly 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?

For a one-parameter tool with no output schema, the description provides a complete enough picture: what it checks, when to use it, and the required authentication context. Minor omissions are the exact return shape and a clearer read-only guarantee.

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 input schema already fully documents the single parameter (api_key with 'Your API key') at 100% coverage. The description adds no additional parameter detail, meeting the baseline of 3.

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 identifies the verb ('Check') and the resource ('remaining credit balance'), making the tool's function unambiguous and distinct from sibling tools like topup_credits or purchase_data.

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?

It gives explicit context for when to use the tool ('Use after purchases'), which helps route an agent correctly. It does not explicitly list when not to use it or alternatives, but the API list makes the distinction reasonably clear.

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