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

Annotations already declare this is a read-only, idempotent, non-destructive operation. The description adds useful domain behavior beyond the annotations: server burns spend the burn price, which is higher than transcription, and the client render path does not spend balance.

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 three short sentences with no wasted text. Purpose is front-loaded, and the following two sentences add spend-model context that is directly relevant to interpreting the returned balance.

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?

Although there is no output schema, the description explains that the balance is returned in cents and in transcription-equivalent audio seconds. Combined with the annotations, this is sufficient for a no-parameter read tool.

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 input parameters, and the description correctly does not attempt to explain any. The baseline for a no-parameter tool is 4, and there is no parameter ambiguity to resolve.

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 gives a specific verb and resource: 'Read the prepaid balance for this API key.' It also states the units returned (cents and transcription-equivalent audio seconds), making it clearly distinct from sibling tools like get_usage or buy_credits.

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 implies this tool is used to check available prepaid credits, and it explains that server burns consume balance while the client render path does not. However, it never explicitly says when to call this instead of alternatives such as get_usage or before buy_credits, so usage guidance remains implicit.

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