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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds value beyond them: the call is free, it behaves differently with and without an API Key (returning data vs. a configuration hint), and it discloses the return payload. That auth-dependent branching is exactly the kind of behavior annotations cannot express.

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?

Three short sentences, front-loaded with what it does and what it costs, followed by the return contents and then the call-timing advice. Nothing is padded and the ordering puts the decision-relevant facts first.

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 zero-parameter, read-only tool with an output schema, the description covers purpose, cost, auth behavior and call timing, which is close to complete. The one gap is that it repeats return fields already documented in the output schema, and it never mentions the related credits_statement sibling.

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?

There are zero parameters and full schema coverage, so the baseline is 4. Nothing about parameters needs explaining, and the description correctly spends no words on input arguments. It instead clarifies the implicit precondition (an API Key must be configured), which is the only input-like concern here.

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 specific verb and resource: check the current API Key's prepaid credit balance and usage. It also enumerates the returned data (remaining credits, RMB equivalent, monthly usage, per-tool prices), so the agent knows exactly what this tool yields. It does not distinguish itself from the sibling credits_statement, which is the only thing keeping it from 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?

It gives a concrete trigger: call this before batch-pulling paid data to confirm the budget is sufficient. That is real when-to-use guidance. It offers no when-not conditions and does not name credits_statement as the alternative for detailed statements, so routing between the two is left to inference.

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