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

Adds real context beyond the annotations: the payment happens on Stripe, the server never sees the card, the API key is retrievable exactly once after payment, and no API key is required to call. It does not warn that a repeat call creates a second checkout session (idempotentHint is false), which is the one behaviorally significant omission for a non-idempotent purchase flow.

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?

Four dense sentences with no filler: what it does and returns first, then the ordering/consent precondition, then the email constraint, then the auth note. Every sentence carries information an agent needs to act.

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 two returned URLs and the one-time key delivery. For a non-idempotent, open-world mutation it stops short of covering failure modes (e.g., what a duplicate call does or what happens if the status_url is polled before payment).

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 coverage is 100%, so the baseline is 3, but the description adds genuine semantics: the email is the receipt and sign-in address, and accept_terms must reflect an actual ToS agreement including section 8. The plan parameter is cross-referenced to list_plans rather than described in the prose, which keeps it just short of a 5.

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 ("Start buying a Commercial API plan") and immediately describes the concrete outputs (Stripe checkout_url and status_url). It is clearly separable from list_plans, which it names as the upstream step, and from the read-only siblings search_cards/get_card_prices.

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?

Explicit preconditions: only call after the user has picked a plan from list_plans and agreed to the terms (accept_terms: true), and use the user's own email because the receipt and account sign-in go there. It also names the alternative path (paging search vs. price files) and states it works without an API key, leaving nothing to inference.

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