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

Beyond the annotations (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false), the description discloses critical non-obvious behavior: the tool never charges anything itself, the agent cannot pay on the user's behalf, and credits land in seconds after payment. It also enumerates each plan with its price and credit allotment, which is behaviorally relevant for choosing arguments.

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-loaded with the usage trigger and the core return value, then the branch logic and recovery steps. The inline plan/price enumeration is dense and slightly interrupts the flow, but it earns its place by letting the agent choose a plan without another lookup.

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 2-parameter, non-required-arg tool with no output schema, the description covers what is returned (a Stripe Checkout URL), what it does not do (charge), the two parameter regimes, and the post-payment verification loop. Nothing an agent needs to invoke it correctly is missing.

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 nonetheless adds meaning by tying 'plan' to the subscription branch and 'topUpUsd' to the existing-plan branch, and by listing plan names with prices/credits. It does not restate the 10–500 USD bounds or default values, which the schema already covers.

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 uses a specific verb and resource: it returns a Stripe Checkout URL for the user to pay through, and explicitly distinguishes itself from the sibling get_credit_balance (which is for verification, not payment). An agent can tell instantly that this is a billing/payment-link tool rather than one of the ~30 image-processing siblings.

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?

The trigger is stated up front: 'Use when a tool fails for lack of credits or a plan.' It then splits the two usage branches explicitly (no active plan = subscription, existing plan = one-time top-up) and gives a post-payment procedure (check get_credit_balance, then re-run the failed tool). This is explicit when-to-use guidance with a named follow-up alternative.

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