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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is fully covered by structured fields. The description adds the returned field inventory (plan name, price, status, limits, credit balance, reset date), which is modest extra context but not behavioral depth like auth or rate-limit details.

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 em-dash-delimited sentence that front-loads the verb and resource and packs the return-value enumeration efficiently. No filler, every clause earns its place.

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 usefully compensates by listing the fields the caller will receive, and with zero params there is nothing else to document. It could be slightly stronger by noting the no-argument nature or routing to list_plans, but it is adequate for this simple read tool.

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 per the rubric this is a baseline 4. There is no parameter semantics to get wrong or 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?

States a specific verb+resource ('Get the user's active billing plan details') and enumerates exactly what fields come back, so the agent knows this is a single-entity read rather than a catalog listing. It does not explicitly name list_plans as the contrasting sibling, so differentiation is left to inference.

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?

The word 'active' implies this is the tool for checking the current subscription state, but there is no explicit when-to-use statement and no mention of list_plans (catalog) or upgrade_plan (mutation) as alternatives. Usage is only implied.

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