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

No annotations are present, so the description carries the behavioral burden. It discloses that the call is free and tied to the API key, and the read-only nature is implied by the content described. It does not mention rate limits or other operational details, but for a zero-parameter usage lookup, the disclosed information is reasonably sufficient.

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 very concise, with no redundant filler. Each phrase adds information: the target resource, the data returned, and the cost characteristic. It could be slightly improved by using a complete sentence with an explicit verb, but it remains efficient.

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, no-output-schema tool, the description lists the main expected fields: plan, credits used, granted, and remaining. It does not specify response format, but that is not essential here given the tool's simplicity and the absence of parameters.

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, so there is no parameter semantics gap for the description to fill. The baseline of 4 applies because the schema is empty and no parameter-level explanation 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 indicates the resource being inspected: the presented API key's plan and credit usage. It is distinguishable from sibling data-query tools because it deals with account credits rather than external data sources. However, it is phrased as a noun phrase rather than an explicit verb statement like 'returns' or 'retrieves'.

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 phrase 'for the presented API key' gives context about what the call scopes to, and 'Free to call' implies it can be used without consuming credits. It does not explicitly state when to choose this tool over siblings or mention any exclusions, though the distinction is largely implied by the tool's purpose.

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