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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description adds real value by disclosing the required auth scope 'orders:read' and framing the result as a 'durable record' plus account-level paid-through time. It does not say what happens when the purchase_id is unknown or unauthorized, which keeps it below 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 short sentences, front-loaded with the resource and followed by the scope prerequisite. Every clause carries information; nothing is redundant with the name or title.

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 helpfully summarizes what is returned: the durable purchase record and the account's paid-through time. It is nearly complete for a single-parameter read, with only the purchase_id provenance and not-found behavior left unstated.

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?

Schema description coverage is 0% for the single parameter, so the description carries the burden. It does add meaning by implying purchase_id refers to a plan-period purchase rather than a credit or board-pass purchase, but it never says where to obtain that ID (e.g., from buy_plan_period or list_orders) or how invalid IDs are handled.

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?

Specific verb ('Retrieve') plus a precise resource ('durable record of a plan-period purchase and the account's paid-through time'). The word 'plan-period' implicitly separates it from get_credit_purchase, get_board_pass_purchase, and get_order, though it never names those siblings explicitly.

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 prerequisite scope 'orders:read' is stated, which is useful context for invoking it. But there is no guidance on when to prefer this over get_order, get_credit_purchase, or list_orders, and no statement of when not to use it. Usage is only implied by the resource name.

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