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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description is free to add behavior: the exact output content and the period-over-period comparison, plus the non-obvious policy that the agent must explain limits and never offer purchases. It stops short of describing defaults or error behavior.

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-loads the resource and scope in the first sentence, then handles the parameter flag and the policy constraint compactly. The metric enumeration is dense but each item earns its place by defining the return content.

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 return values need not be explained, yet the description still characterizes them well. The only meaningful gap is the unspecified default for the optional period parameter, which an agent would need to know to call it predictably.

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?

Schema coverage is 0%, so the description must compensate. It fully explains previous=true (last closed period vs. in-progress) and enumerates what the period selection yields, though the week/month enum itself is self-evident from the schema and the default period for this 0-required-param tool is never stated.

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 (read) and resource (account usage) with the exact scope of metrics returned: credits, images, video/audio seconds, per-model credits, prior comparable period, and 30-day daily credits. This distinguishes it clearly from sibling get_credits, which reads a balance rather than a period's spend/earn breakdown.

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 conditional semantics for previous=true and an operational directive ('Explain limits; never offer purchases'), but never states when to choose this tool over get_credits or get_account_context. Usage is implied rather than routed.

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