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

The description claims the tool is 'idempotent', but the annotation idempotentHint is false. This is a direct contradiction. While the description adds useful context about Stripe collecting card details outside chat and the URL not completing the purchase, the idempotency mismatch is a critical behavioral inconsistency that misleads the agent. Per scoring rules, a contradiction drops this dimension to 1.

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 sentences with no redundancy. The usage condition is front-loaded, followed by the action and a clarifying note about external payment handling. Every sentence earns its place and the description is appropriately compact.

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?

The description covers the essential context: prerequisites, what the tool does, and the return value. It lacks details on error handling or edge cases, but for a checkout-creation tool with full parameter documentation, this is sufficient. The idempotency contradiction slightly undermines completeness, but that is already penalized in the behavioral dimension.

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 the baseline is 3. The description does not add per-parameter detail beyond the schema; it references the flow but leaves parameter specifics to the schema. Since the schema already thoroughly documents each parameter (e.g., confirm const, plan_id pattern), the description adds no extra semantic value for parameters.

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 clearly states the action ('creates an idempotent Stripe-hosted subscription Checkout'), the resource it acts on, and the return value (external checkout URL). It positions the tool within a sequence (after email verification and plan selection), distinguishing it from siblings like list_subscription_plans and verify_email_signup without needing explicit naming.

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?

Explicitly states 'Use this only after email verification, a plan selection from list_subscription_plans, and explicit user confirmation.' This gives clear preconditions and the context in which the tool should be called. It also clarifies that creating the URL does not complete the purchase, preventing premature usage.

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