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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds genuinely new behavioral facts: the call is free of credit cost and its output branches on whether an API key is presented. That auth-dependent behavior is exactly the kind of context annotations cannot express.

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?

A single dense sentence with no filler; the primary case (keyed usage) leads and the keyless fallback follows. Every clause earns its place and nothing is repeated from the title or 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?

With no output schema, the description does carry the burden of describing return content, and it names the fields returned (plan, used/granted/remaining). It does not state units, reset periods, or error behavior when a key is invalid, but for a zero-parameter read call this is close to sufficient.

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 the baseline is 4 and there is no parameter semantics for the description to compensate for. It correctly spends no words on arguments.

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 names the exact data returned (plan, credits used/granted/remaining) and the conditional branch when no API key is presented (preview calls left today). It implicitly separates itself from buy_credits/request_api_key by being a read of existing state, but never names a sibling explicitly, so it stops short of a 5.

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?

'Free to call' and the with-key/without-key branch imply that this is the zero-cost way to check quota before spending credits, but usage is only implied. There is no explicit when-to-use instruction, no exclusions, and no named alternative such as buy_credits for topping up.

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