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

Annotations already declare readOnlyHint=true, so safety is covered; the description adds the actual payload semantics (trial days remaining, next billing date) and a self-throttling rule for proactive calls. It stops short of mentioning auth requirements or caching/staleness of billing data, which keeps it from 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?

Two sentences, both load-bearing: the first defines the output, the second defines the trigger. The bilingual gloss ('período de prueba') plausibly aids matching Spanish-language user phrasing rather than padding.

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?

A zero-parameter read tool with no output schema, fully covered: the description explains what comes back (status values, days remaining, billing date) and when to call it. Nothing an agent needs to invoke it correctly 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, so there is nothing to disambiguate and the schema is trivially complete. Baseline 4 applies; no additional parameter guidance is owed.

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?

A specific verb and resource ('Get the account's subscription status') followed by an explicit enumeration of the returned states (trial, active, past_due, canceled). No sibling tool covers subscription/billing status, and the description makes that scope unmistakable.

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?

States two concrete triggers: when the user asks about their plan, trial or billing, and proactively once per session if unsure about payment proximity. The 'once during a session' qualifier is an explicit usage constraint that prevents over-calling.

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