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

Annotations already indicate readOnlyHint=false and idempotentHint=false, so the agent knows it's a mutating operation. The description adds the key behavior that it does not charge anything, and returns a continue_url for payment. This provides useful context beyond the annotations.

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?

Two sentences with no fluff. The first sentence states the action and the second explains the return and the no-charge behavior. Information is front-loaded and every word earns its place.

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?

The description covers the core return value and clarifies it does not charge, but it omits details on offer_id choices and quantity semantics. Given the enum of offers and the quantity parameter, an agent may not know which offer to select or what quantity means. The description is adequate for the basic flow but incomplete for correct parameter selection.

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%, so the description must compensate for parameter semantics. The description mentions 'one offer' which implies offer_id selects the offer, but it does not explain the enum values or the quantity parameter. The agent is left without guidance on how to choose offer_id or interpret quantity, a significant gap given the enum has many options.

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 clearly states the action: 'Start a purchase of one offer' and specifies the resource (offer) and the return (continue_url). It also clarifies that it does not charge anything, distinguishing it from any charging operation. This is a specific verb+resource with clear scope.

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 description implies usage when initiating a purchase, but it does not explicitly state when to use this tool versus alternatives like get_checkout or list_products. No exclusions or alternative conditions are provided. The context of starting a purchase is clear, but the routing to siblings is left to inference.

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