billing_usage

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

No annotations are provided, so the description must carry the behavioral load — and it does disclose the trait that matters most here: 'Read-only; it never spends.' For a wallet-adjacent tool this removes the obvious agent concern about incurring cost. It stops short of stating auth requirements or whether results are cached/live, which keeps it from a 5.

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?

One sentence, front-loaded with the trigger ('before expensive runs'), then the payload list, then the safety guarantee. Every clause earns its place with no 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?

With no annotations and no output schema, the description compensates by listing what the caller gets back and by asserting the read-only/no-spend property. For a zero-parameter read tool, nothing an agent needs in order to call it correctly 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 the baseline is 4; there is nothing for the description to clarify beyond the empty schema. The enumerated return fields add value that the schema cannot supply, since no output schema exists.

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?

States a specific verb and resource ('Check the caller's wallet') and then enumerates the exact data returned: credit balance, tier, storage used vs quota, turns used, spend by product. With no overlapping sibling among the listed tools, no differentiation is needed, and the enumeration makes the scope unmistakable.

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?

Gives an explicit trigger condition ('before expensive runs'), which tells the agent when to reach for this tool rather than discovering limits mid-task. There are no alternative billing tools to exclude, so the absence of a when-not clause is not a real gap.

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