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

Annotations already cover the safety profile (non-readOnly, openWorld, non-idempotent, non-destructive). The description adds real operational context beyond them: this call only initiates a checkout session and hands off to the browser, so the agent learns the payment is not completed server-side. It still doesn't warn that repeated calls spawn multiple sessions or what happens to an existing subscription.

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 tight sentences, front-loaded with the action and the returned artifact. Every clause earns its place, including the browser handoff instruction.

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?

For a one-parameter mutation with no output schema, the description supplies the key return value (checkout_url) and the follow-up step, and annotations cover the safety profile. Minor gaps remain around idempotency/duplicate subscriptions and auth prerequisites.

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% and there is a single enum-constrained parameter, so the schema carries the meaning. The description's mention of 'the Pro plan' loosely maps to the plan_id enum, but adds no format, default, or constraint detail beyond the schema — baseline 3.

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 concrete verb and outcome ('Create a Stripe Checkout session for the Pro plan and return checkout_url'), so an agent knows exactly what this tool produces. It doesn't explicitly contrast itself with siblings like get_plan, list_plans, or get_usage, which is the only thing keeping it from a 5.

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?

'Open it in the browser for the user to pay' gives an implied post-call workflow, which is useful. However, it never states when to choose this over get_plan/list_plans or what preconditions (e.g., already on Pro, signed in) make it appropriate, so usage is only implied.

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