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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds real context beyond that: the balance is 'spendable', usage is measured 'against quota' for 'this billing period', and credits have a reset date — scoping the data the agent will receive.

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?

Front-loaded with the core purpose, then the distinguishing detail. Slightly redundant — 'this billing period's usage against quota' re-treads the opening line — but each sentence carries routing or scoping value.

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?

An output schema exists, so the description needn't document return structure, yet it still signals the shape of the response (balance, quota usage, reset date). Zero-param, read-only, no auth caveats left unstated — nothing needed 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 there is nothing for the description to disambiguate; baseline 4 applies. Nothing in the text conflicts with the empty schema.

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 resource (credit balance) and its three components — remaining credits, period usage, reset date — with a concrete verb. It also explicitly carves itself apart from estimate_run by naming the cost-estimation sibling it is not.

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?

Explicitly frames the question it answers ('how many credits are left' without pricing a run) and names the alternative (estimate_run) plus the condition that selects it (what a run will cost / whether balance covers it). No inference required.

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