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

Annotations already declare readOnly, idempotent, non-destructive behavior, so the bar is lower. The description adds context not present in structured data: the call is free and costs 0 credits, which materially affects an agent's willingness to call it preemptively, plus it previews the returned trial-state fields.

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 sentences, both front-loaded: what is returned first, then the decision rule and the zero-cost fact. Nothing is repeated from the title or the annotations.

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-only utility with no output schema, the description supplies everything needed: the returned fields, the cost, and the recommended moment to call it. No structural 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?

The tool takes no parameters, so the schema-coverage baseline of 4 applies and no parameter explanation is needed. The description correctly spends no words on arguments and instead focuses on what the call yields.

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, credits remaining and trial status" names a specific verb and resource and enumerates the exact fields returned, including trial end date, days left, and whether the trial has started. It is unmistakably distinct from the sibling TikTok/X data-fetching tools.

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?

"Use it before a batch of calls if you are unsure what the key has left" gives an explicit trigger condition tied to a concrete decision. It stops short of stating when not to call it or naming an alternative for related questions, but for a zero-parameter utility that gap is minor.

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