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

Annotations already declare readOnly/idempotent/non-destructive, yet the description adds real substance beyond them: API-key authentication requirement, no quota consumption, no rate-limit headers emitted, and the period being the UTC calendar month. These are exactly the operational details an agent needs and cannot infer from annotations.

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 most decision-relevant facts (free, needs API key, what it returns) and short overall. Slight redundancy: 'The account's current-month usage and quota' largely restates the preceding sentence and could be merged.

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?

For a zero-parameter read tool with full annotation coverage and an output schema, the description covers everything an agent needs: cost, auth, side effects, and the period definition. Return values are handled by the output schema, so nothing material 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 no parameter semantics to document; the baseline for a parameterless tool applies. The description correctly avoids inventing inputs.

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 ('Shows the plan, the calls used and left this month, and when the period ends') with concrete returned fields. It is unmistakably distinct from all siblings, which are optimization/validation tools, so an agent can route correctly without opening the schema.

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?

Provides clear call context: it is free but requires an API key, and it does not consume quota, which tells the agent it is safe to call at any time. It stops short of naming explicit when/when-not conditions or sequencing relative to the optimize_* tools, so it is not a 5.

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