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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds real behavioral context beyond that: that the plan is purely informational and never causes a refusal, plus null semantics for 'userName' and trial 'termEnd'/'daysLeft'. It does not discuss error/auth-failure behavior, so not a 5.

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-loading is good - the core purpose opens the description - but the remaining text is a dense run-on packed with nested parentheticals that makes field semantics hard to scan. Information density is high and most content is justified given the absent output schema, but the structure could be broken into cleaner clauses.

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 and no parameters, the description must carry the full burden of return-value explanation, and it does: it names the fields, describes the subscription object's members, explains null cases, and disambiguates key id vs user id vs project id. Nothing an agent needs to interpret the response 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, which sets the baseline at 4 by the rubric. The description uses its space instead to document return-value field names and null conditions, which is appropriate given there are no inputs to explain.

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?

Opening clause 'Reports the current API key' gives a specific verb and resource, immediately followed by the concrete payload (authenticated user, project/team, plan). It is unmistakably distinct from siblings like project_get or stats_fetch, which is reinforced by the explicit id-disambiguation against those 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?

There is no explicit when-to-use statement, but the description does route the agent across tools by noting that the response's 'user' field is the entityId the stats tools want, and by clarifying that riddle_get's 'plan' is a different concept. This is useful implied guidance, but the basic 'call this to identify the connected account' framing is left to inference.

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