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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds substantial context beyond that: the unit (millionths of a dollar), the price basis (X's published prices), the aggregation (by operation), the billing window (UTC month), and the pass-through-at-cost policy. It omits rate limits and any freshness/delay in reporting.

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?

Two tight sentences, front-loaded with the core purpose and unit before the pricing-policy note. 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?

There is no output schema, so the description must carry the return-value burden. It communicates the unit, price basis, and grouping (by operation), which is most of what an agent needs, but it does not sketch the response shape (e.g., a per-operation mapping vs list) or timing of the data.

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; the baseline for a parameterless tool is a 4. Nothing is missing or misleading on this dimension.

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 (return) and resource (X API call costs) with precise scope: this UTC month, broken down by operation. It is clearly distinguishable from every sibling, none of which deal with cost/usage reporting.

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 purpose implies when an agent would call it (billing/cost checks), but there is no explicit when-to-use guidance, no prerequisites, and no named alternative among the many sibling tools. Usage is only implied.

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