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

Annotations already establish readOnly, idempotent, non-destructive and closed-world behavior, so the bar is lower. The description adds genuinely useful semantics beyond them: purchases last 30 days with no auto-renewal, and the account field is null when unauthenticated, which tells the agent how to interpret a partial response.

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?

The sentence 'Each purchase lasts 30 days, with no automatic renewal.' is duplicated verbatim, which is wasted space in an otherwise two-sentence description. The substantive content is front-loaded and tight apart from that repetition.

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 carries the return-value burden and does so: it enumerates price, term, and active plan, and explains the null-account case. Ordering, currency, and the shape of the plan objects are left unspecified, but the essentials for correct invocation are present.

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 no schema surface for the description to compensate for. The description correctly adds no parameter commentary.

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 resource and its contents precisely: plans for sale with price and term, plus the account's active plan. That distinguishes it from the mutation siblings buy_plan and cancel_plan, though the implicit overlap with the 'pricing' sibling is not addressed.

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 statement of when to call this versus alternatives such as pricing or buy_plan, and no mention of what precedes a purchase. The only contextual hint is that a null account is returned without credentials, which is incidental rather than usage guidance.

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