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

Annotations already establish this as a read-only, idempotent, non-destructive operation. The description adds useful context about history start date and dataset-dependent coverage, though it does not mention authentication behavior or what happens with an invalid key.

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?

The description is short and front-loads the returned information before the caveat and sibling pointer. The first sentence is a fragment rather than a direct statement, but no sentence is wasted.

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 read-only status tool with rich annotations and no output schema, the description covers the key returned concepts and data coverage caveats. The remaining gap is that the sole API key parameter is never explained.

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?

There is one parameter, api_key, and schema description coverage is 0%, so the description needs to compensate. It never mentions the API key, its required status, or its purpose, leaving the only parameter entirely undocumented in text.

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?

The description clearly states the information returned: purchased scope, expiry, and remaining calls. It lacks an explicit verb, but an agent can identify it as a plan/subscription status tool and distinguish it from days_available via the explicit routing note.

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?

It gives a clear usage context for plan status and explicitly points to days_available for the exported UTC day range. It does not provide explicit when-not conditions, but the sibling handoff is clear enough for correct selection.

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