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

Annotations supply only readOnlyHint=true; the description carries the real behavioral load by disclosing free-tier pool size, midnight UTC reset, the 20 req/min rate limit, the 2-concurrent cap, and Pro fair-use throttling thresholds with the notable assurance that usage is never blocked or billed.

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 one-line purpose, then dense supporting detail in two sentences with no filler. Some of the enumerated limits (5,000/day, 10M/30 days, 30 req/min) partly restate what the output schema already returns, so a small amount of space is not strictly earned.

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 zero parameters and an output schema covering the return shape, the description needs only to orient the agent about what the response means. It does that and then some, so nothing needed for correct invocation 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 explain and the baseline is 4. The description correctly does not invent inputs, though it also makes no statement about how the calling key is identified (implicitly via auth).

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?

"Check this API key's plan and usage" names a specific verb (check) and resource (plan + usage) and is unambiguous against siblings like text_to_speech or clone_voice, which all perform generation rather than introspection.

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?

Usage is only implied: an agent can infer it should call this to learn quotas or plan, but the description never states when to call it, when not to, or any alternative (e.g., get_job_status for job progress). No explicit routing guidance is given.

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