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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and destructiveHint false, so the safety profile is covered. The description adds useful behavioral context by specifying that the API key is taken from the HTTP header and that passing a key as an argument is forbidden. No contradiction with annotations.

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 short sentences with no filler. The core purpose is front-loaded, and the critical 'never pass a key' warning is included without bloating the description.

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 balance check, the description is complete. It explains where the API key comes from, warns against passing arguments, and the annotations cover idempotency and non-destructiveness. No output schema is present, but this requires no special return-value explanation.

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 is empty, so the baseline is 4. The description reinforces that no key argument should be passed, which clarifies the intended zero-parameter usage beyond the schema itself.

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 the tool returns a balance check for the API key already present in the connection's HTTP header. It uses a specific verb ('balance check') and resource ('API key'), and makes the zero-argument nature explicit, distinguishing it from any key-management or purchase siblings.

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 gives clear context: use this to check the balance of the connection's API key without passing credentials. It does not explicitly name alternative tools or state when not to use it, but the instruction 'Never pass a key as an argument' provides practical invocation guidance.

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