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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds useful behavioral context beyond annotations by specifying the exact return contents and units (integer cents, formatted amount, free invoices left). It does not mention auth or rate limits, but for a simple read-only balance lookup that is not a major gap.

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 sentences, front-loaded with the return values and followed by the intended usage. Every sentence earns its place; there is no repetition 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?

There is no output schema, so the description must explain return values, and it does so by naming all three returned pieces of information and their units. For a zero-parameter, read-only tool, nothing essential is missing.

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 parameter semantics carry no burden. The baseline for zero parameters is 4, and the description correctly does not waste space describing inputs.

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 states a specific verb and resource (returns the account's prepaid credit balance) and explicitly lists the returned values: integer cents, formatted amount, and free invoices left. This is clearly distinct from all sibling invoice/customer tools, which manage records rather than account credit.

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 a concrete usage context: 'Use it before finalizing an invoice to check the charge can be covered.' That is clear guidance for when to call it. It does not name exclusions or alternatives, but no sibling tool provides the same balance information, so the omission is minor.

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