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

Goes well beyond the annotations by disclosing the side effect (a Stripe session is created and a URL returned), the hard constraint that checkout cannot be driven programmatically, and three distinct outcome branches: normal URL, error for live paid subscribers, and URL plus already_premium_note for comped/beta Premium users. Annotations only state the generic non-read-only, non-destructive profile.

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-loads the core action and the return, then uses clearly labeled Args/Returns blocks for the details. No sentence is filler; even the edge-case notes earn their place by preventing wrong invocations.

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?

There is no output schema, and the description compensates by spelling out the return keys (checkout_url, plan_name, error, already_premium_note) and the conditions producing each. Combined with the paid-plan constraint and the browser caveat, an agent has everything needed to call 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 0%, so the description must carry the parameter meaning — and it does: plan_id is a SubscriptionPlan.id, restricted to paid plans. It does not say where to obtain a valid plan id (e.g. list_subscription_plans), which is the one remaining gap for a 1-param tool.

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 and return the URL') and makes the scope explicit — the URL is for a browser, not programmatic use. This clearly separates it from siblings like open_billing_portal, list_subscription_plans, or cancel_subscription.

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 substantive context: plan_id must be a 'paid plan only', and a user with a live paid subscription will get an error pointing at the billing portal — effectively steering existing subscribers elsewhere. It stops short of naming the alternative tools (open_billing_portal, list_subscription_plans) explicitly, so it is strong but not fully routing.

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