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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the agent knows this is a safe read operation. The description adds a meaningful qualifier ('purchased credit balance' as opposed to free or promotional credits) and clarifies the value is 'available to the API,' but it does not describe any further behavior such as caching, rate limits, or response units. With annotations carrying the safety profile, a 3 is appropriate.

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 a single sentence with no filler words. It immediately states the core subject and provides a clarifying qualifier ('purchased' and 'available to the API'). Every word earns its place.

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, the description is nearly complete. It identifies what is returned (credit balance) and its scope (purchased, API-available). The only missing detail is the exact return format (e.g., integer, float, string), and no output schema exists to fill that gap, so a small deduction is warranted.

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, so the schema carries no burden and the description does not need to explain parameters. The description itself adds semantic meaning beyond the empty schema by defining what the balance refers to. Baseline 4 is justified for a zero-parameter tool.

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 ('account's purchased credit balance') and the operation (available to the API), and it is obviously distinct from the sibling tools, which all involve documents or translation. Even though the sentence is a noun phrase rather than a verb, it unambiguously conveys that the tool retrieves credit balance.

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 no guidance on when to use this tool versus alternatives. There is no mention of checking credits before other operations, no prerequisites, and no stated conditions for use. The context signals show no siblings that compete directly, but the description still fails to provide any usage direction.

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