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

Annotations already declare readOnlyHint and idempotentHint, so the safety profile is covered. The description adds a genuinely useful behavioral trait beyond the annotations: 'Free — no credits consumed,' plus the QUOTA_EXCEEDED failure mode. It stops short of noting rate limits or auth scope.

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 purpose and the cost/error guidance, and the example prompts are a reasonable affordance. The inline 'Returns: {...}' block is somewhat redundant given a separate output schema exists, adding length without new information.

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 simple zero-arg read tool with an output schema, the description supplies everything an agent needs: purpose, cost behavior, when to call it, and a preview of the return shape. No meaningful gaps remain.

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?

Zero parameters, so there is no parameter semantics to document; the baseline for a no-arg tool is 4. Nothing in the description contradicts or complicates 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 verb+resource: 'Get current credit balance and plan details for your API key.' It is clearly distinguishable from siblings like extract_structured or summarize_document, which consume credits rather than report on them.

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 tells the agent when to call it ('Check this before running credit-consuming operations (extract, summarize, etc.)') and why ('to avoid QUOTA_EXCEEDED errors'). It names the relevant sibling categories, leaving nothing to inference.

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