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

Annotations already mark it read-only, idempotent, and non-destructive. The description adds meaningful context: it returns a specific JSON shape, never exposes the full card number, requires the API key, and notes where plan changes do occur. It does not contradict the 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?

The description is dense but well-organized: return payload first, usage conditions next, exclusions with named siblings, then output keys, safety, and auth. Every clause adds information an agent needs; nothing is redundant or filler.

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 inspection tool with no output schema, the description is complete: it states what data is returned, when to call it, what not to use it for, how to distinguish it from related tools, and the auth requirement. An agent has everything needed to select and invoke it correctly.

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 the description correctly avoids parameter-level detail. The 'Needs your API key' line is an authentication context note rather than an input parameter. Baseline 4 applies for zero-parameter tools with no missing semantic burden.

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 resource ('billing state of your account') and enumerates the contained information. It explicitly contrasts itself with get_usage, get_statement, and list_plans, making sibling differentiation immediate and unambiguous.

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 concrete when-to-use triggers: before buy_pack and when a call is refused for budget reasons. It also names the alternatives for usage numbers, invoices, and plan prices, and states that plan changes happen in the web console rather than through this tool.

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