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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered. The description adds genuinely new behavioral context: the call is free of charge and its output branches on whether an API key is presented. That auth-dependent branching is exactly what 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 compact sentence front-loads what is returned, then the key-dependent branch, then the cost note. Every clause carries information and there is no filler.

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 with no output schema and full annotation coverage, the description covers what is returned, the two behavioral cases, and cost. Minor gaps around the shape of the returned values are acceptable since annotations already carry the safety profile.

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 are no parameters, so the baseline is 4. The description correctly conveys that behavior is driven by the ambient API key rather than any argument, which the empty schema cannot express on its own.

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 resource returned (plan, credits used/granted/remaining) and the conditional case without a key. It is clearly distinct from sibling auth tools like claim_api_key and request_api_key, which manage keys rather than report usage. It stops short of a 5 only because it doesn't explicitly frame itself against any alternative.

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 implies the two usage contexts: with a presented API key versus without one (preview calls left today). It does not state when to call this versus checking usage elsewhere or mention any rate limits around the preview. The guidance is implied rather than explicit.

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