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

Annotations only declare readOnlyHint=false, destructiveHint=false and openWorldHint=true. The description adds the critical behavior the annotations cannot convey: nothing is charged until confirmPaymentSpend is true, the price is fetched from Polar on the first call, and payment is completed on Polar's external page rather than in this tool.

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 tight sentences, front-loaded with the outcome and then the two-step mechanism. Nothing is padded and each clause carries information the agent needs before invoking a payment flow.

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 only two parameters, no output schema, and an open-world payment side effect, the description covers what is missing from structured fields: the two-call protocol, the charge-free first call, and where the actual payment is transacted. Nothing an agent needs in order to call this safely is absent.

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 (plan and confirmPaymentSpend) are already documented in the schema, and the enum for plan is self-explanatory. The description reinforces the confirmPaymentSpend semantics but adds no syntax or format detail beyond what the schema states, so the baseline of 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?

The description opens with a specific verb and named outcomes: 'Upgrade to Pro or Premium.' The resources (the two plans) are named and the operation is unambiguous, so an agent can tell exactly what this tool accomplishes without opening the 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?

It gives explicit two-step procedural guidance: call once without confirmPaymentSpend to read the exact price, then call again with confirmPaymentSpend: true to get the Polar checkout URL. It also explicitly states when no charge occurs, which is the key when-to-use versus when-not distinction for a payment tool.

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