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

Annotations only give generic hints (readOnlyHint=false, openWorldHint=true); the description adds the critical non-obvious behavior that calling it charges nothing and costs no credits, that the link is pre-tied to the key's account so no sign-in is needed, and that credits land within seconds. These are exactly the facts an agent needs to avoid mis-describing a purchase operation to a user.

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 action and the returned artifact, then flows through usage, timing, and cost. Slightly redundant at the tail: 'Calling this charges nothing' and 'Costs no credits' convey the same fact twice.

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 one-parameter mutation tool with an output schema present, the description covers everything needed: what it does, what it returns, that it is free, how the payment completes out-of-band, and how to confirm success. No gaps remain for correct invocation.

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?

The single parameter has 100% schema description coverage ('How many credit packs to buy, 1 to 10. Defaults to 1'), so the schema already carries the semantics. The description's '1 to 10 credit packs' merely restates that range without adding format or selection guidance, making this baseline-3 territory.

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 ('Buy more credits for your API key') and immediately names the artifact produced ('a Stripe Checkout link'). It is clearly distinguishable from every sibling, especially check_credits, which it explicitly routes to for confirmation.

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?

Explains exactly how to use the result ('open the link in a browser if you can, or give it to the user'), what the user must do to complete payment, and names check_credits as the follow-up verification step. Both the usage context and the alternative verification flow are explicit.

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