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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, but the description adds real value: the 404 error behavior for unknown IDs, the livemode test-vs-live distinction, and the explicit statement that cancellation/refunds must happen elsewhere. It stops short of describing pagination or caching, though neither is relevant for a by-ID fetch.

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?

Front-loaded with the core action, followed by routing guidance, error semantics and examples. The four example utterances are slightly more than needed but each illustrates a distinct use, so the cost is low.

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 read-only lookup with an output schema needs little beyond routing and error behavior, both of which are present. An agent has everything required to select and invoke 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?

Schema description coverage is 100%, so both parameters are already documented in the schema. The description's 'Pass livemode: false to read test-mode records' largely repeats the schema text, though the example uses add mild illustrative value. Baseline 3 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 and resource: 'Retrieve a single subscription by ID from your Checkout Page account.' The singular 'single' paired with the ID requirement clearly distinguishes it from list_subscriptions and get_subscription_payment.

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?

Explicitly routes the agent: use list_subscriptions to discover an ID, use prepare_subscription_cancellation for cancellation context, and perform cancellation/refunds in the dashboard. Both when-to-use and when-not-to-use are covered.

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