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

With only readOnlyHint=false and openWorldHint=false in annotations, the description carries real behavioral detail: the operation returns a checkoutUrl rather than completing payment, credits are asynchronous (arrive on payment success), any key of the customer can spend them, and a 409 is returned where payments are disabled. That is exactly the kind of failure-mode and lifecycle disclosure annotations cannot express.

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, each earning its place: what it does, what it returns and when credits land, and the failure case. The checkout/return information is front-loaded rather than buried.

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?

No output schema exists, yet the description names the key return value (checkoutUrl) and the async fulfillment model, plus the 409 condition, which is enough for an agent to call this correctly. Minor gaps remain around currency handling and what the customer sees on return.

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 only 50% (returnUrl is documented, currency is not) and the description says nothing about either parameter – no mention of currency choice or how returnUrl interacts with the hosted checkout flow. It fails to compensate for the coverage gap.

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 ('Open a hosted checkout') and resource ('a pack of ten contract credits'), including the scope of whose credits they are. It is clearly distinguishable from siblings like get_credit_balance, which only reads a balance.

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?

Gives clear context for when this applies: a human opens the checkoutUrl in a browser and credits arrive only when payment succeeds. It does not explicitly name alternatives such as get_credit_balance or state when not to call it, so it stops short of full routing guidance.

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