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

Annotations already declare openWorldHint, non-idempotent and non-readonly. The description adds genuinely new behavior: the result is a checkout_url on the external checkout.stripe.com domain that must be opened in a browser for the user to pay. It does not cover failure/expiry behavior of the session.

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 action, then the constraint, then the return value. No filler; each sentence carries distinct information.

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 correctly explains the return (checkout_url) and where it lives. The one gap is that required_parameters is 0 while plan_id is described as needed to produce a URL, so the no-plan_id behavior is left unspecified.

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% with enums documented for all three parameters, so the baseline is 3. The description repeats the plan_id values ('pro' or 'scale') without adding format or normalization detail beyond the schema.

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: 'Create a Stripe Checkout session for a plan upgrade.' This clearly separates it from siblings like list_plans and get_plan, which retrieve plan data rather than initiate 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?

Gives explicit workflow ('Pass plan_id ... open it in the browser for the user to pay') and an exclusion ('Do not send the user to the marketing pricing page'). It does not, however, point to list_plans/get_plan as the way to discover plans, leaving that routing to inference.

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