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

Beyond the annotations (openWorldHint, non-idempotent, non-destructive), it discloses the crucial behavior that the tool itself performs no charge and that fulfillment is human-driven and asynchronous. It also discloses a side effect not visible anywhere else: a new API key is created after payment and is claimable only once.

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 tight sentences, with the most important correction ('this tool charges nothing') front-loaded ahead of the fulfillment details. No sentence is redundant or restates the title.

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, no-output-schema tool, it covers everything an agent needs: what is produced (a checkout link plus a session_id), who completes it, and how to redeem the result. Nothing essential is left to inference.

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 0% and the single `pack` parameter is documented nowhere in the schema, but the description redirects the agent to list_credit_packs as the source of valid pack identifiers, which resolves most of the ambiguity. It stops short of stating the identifier format or whether the pack determines the price.

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?

Specific verb + resource: 'Create a Stripe Checkout link to buy a credit pack,' and it immediately corrects the misleading name/title by noting 'this tool charges nothing.' It explicitly points to the sibling list_credit_packs for the pack values, so an agent can separate it from claim_api_key and get_credit_balance without opening schemas.

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?

States the precondition (a human must open the link and pay) and branches the follow-up path clearly: with an API key the credits attach to that key; without one, the agent must call claim_api_key with the returned session_id, exactly once. Explicit alternative/next-step routing is present.

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