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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds the scope boundary that matters behaviorally — 'purely about plan identity and billing dates' — preventing an agent from expecting usage counters. It does not add anything on freshness/caching of billing dates, which keeps it short of 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?

Three sentences, front-loaded with the resource and its fields, then usage triggers, then the exclusion. The quoted example questions are the one potentially expendable element, but they directly aid routing and cost little.

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?

Zero parameters, an output schema that carries the return shape, and annotations covering the safety profile — the description supplies exactly the missing pieces (scope and sibling routing) and nothing more is needed to call it correctly.

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 nothing for the description to disambiguate. The field list it does give is return-value content rather than parameter semantics, so no additional credit applies.

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 (return) and resource (user's plan + billing-date info) and enumerates the exact fields returned: tier, renewal date, trial end date, cancel-at-period-end state. It also explicitly delimits itself against the sibling get_plan_limits, so an agent can distinguish the two without opening either schema.

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?

Gives concrete trigger utterances ("when does my plan renew?", "how many days left on my trial?", "is my subscription cancelling?") and an explicit negative routing rule: for quotas and usage, call get_plan_limits instead. Both the when and the when-not are stated outright.

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