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

Beyond the readOnly/non-destructive annotations, the description discloses the exact return fields, that a negative balance is legal, that an unreadable balance means unknown rather than zero, and that payment settlement is asynchronous so a completed charge may not yet show a usable balance. These are precisely the non-obvious behaviors an agent could otherwise misread.

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 core action and return shape are front-loaded, then caveats follow in short declarative sentences. It runs six sentences, and the direct_use_charged detail is somewhat niche, but each sentence carries distinct semantic weight rather than restating the name.

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 carries the return-value burden itself and does so fully: it enumerates fields, the error case, and the alternate direct_use_charged shape. For a zero-parameter read tool, nothing an agent needs to interpret the response 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 takes zero parameters, so there is nothing for the description to explain and the schema is trivially complete. Baseline 4 applies; no parameter-level value can be added.

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 first sentence gives a specific verb and resource ('Read the current AI credit balance') scoped to the authenticated account, and the closing line ('does not initiate a purchase') draws a boundary against payment/purchase actions. It stops short of naming the closest read siblings (get_credit_transactions, get_payment_status), so an agent must infer the split between current balance and transaction history.

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 establishes the read context and explicitly states a when-not ('This tool does not initiate a purchase'), which steers agents away from using it as a purchase trigger. It does not name alternatives like get_credit_transactions for historical detail, leaving that routing to inference.

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