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

Annotations already establish non-read-only, non-destructive, open-world behavior; the description goes further by stating no card is charged and enumerating the exact fail-closed conditions under which no session is created. That is genuine behavioral context beyond the annotations, though it doesn't explain idempotency or what the returned URL looks like.

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 short sentences, front-loaded with what the tool produces and immediately followed by the important non-charging guarantee. Slightly dense jargon ('unavailable covered agents') costs a bit of clarity but nothing is wasted.

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?

An output schema exists, so return values need not be explained, and the failure modes are well covered. The significant remaining gap is parameter meaning: with 0% schema coverage on two required params, the agent has no guidance on how to construct account_ref or which plans are eligible.

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% and neither plan_id nor account_ref is described anywhere. The mention of 'Draft plans' and 'recurring Price IDs' hints that plan_id refers to a plan record, but account_ref is entirely unexplained, leaving the agent to guess its format and meaning.

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 names a specific verb and artifact – prepare a Stripe-hosted subscription Checkout URL – and adds the key disambiguator 'It never charges a card', which separates it from charge/record tools like record_subscription. It does not explicitly name a sibling alternative, so it stops short of 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?

Usage is implied (produce a hosted checkout URL for a subscription plan) and the fail-closed preconditions – draft plan, missing owner approval, unavailable covered agents, missing recurring Price ID – effectively tell the agent what state must hold first. However, there is no explicit statement of when to prefer this over customer_portal_link or record_subscription.

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