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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint, so the description adds value beyond them by stating 'Read-only', the default period, the 'list' mode, that an agent sees only its own usage, and that periods follow subscription cycle or calendar months. It does not mention rate limits or error behavior, but the provided content is strong.

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?

Every sentence carries high-signal information, and the core purpose is front-loaded. It is dense and long, but not redundant; the only slight inefficiency is the extensive return-format enumeration, yet with no output schema that also serves as needed context.

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 output schema, the description enumerates the JSON fields for both normal and list modes, explains default scope, period, authentication need, and read-only behavior. For a single-parameter read tool with clear result shapes, 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?

Schema coverage is 100%, giving the parameter baseline at 3, but the description adds what the schema does not: default period behavior, that 'list' is a special enumerator, and that periods follow the subscription cycle or calendar months. These are meaningful semantic enrichments beyond the schema's start-date example.

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-resource pair ('get statement') and details exactly what it contains: 'invoice facts plus usage grouped by API key or agent, chain and method class'. It also differentiates from siblings explicitly by naming get_usage and get_billing_status, leaving no ambiguity about what this tool uniquely does.

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 explicit when-to-use ('Use it for month-end reconciliation or to show an owner what a period cost') and when-not-to-with alternatives ('Not for live usage (use get_usage) or the card and subscription (use get_billing_status)'). It also adds scoping rules (agent's key sees only that agent's usage) and cycle behavior, giving an agent complete decision guidance.

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