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

The description adds value beyond annotations by stating the tool is 'server-checked', and by describing the return format (new discount/shipping/total or applied=false with reason). Annotations already indicate idempotentHint=true and destructiveHint=false, so the description complements these with behavioral details about validation and error handling.

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?

The description is very concise with only two sentences. However, the phrase 'Args: code' is redundant given the schema, and could be omitted without loss. Still, it is efficiently structured and front-loaded with the core action.

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 simple tool with one parameter and a described output schema (though not shown in detail here), the description covers the essential behavior: validation, application, and return values. It does not mention prerequisites like an existing cart, but this may be implied by the tool's role in a shopping flow.

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 input schema already provides full coverage (100%) with a clear description for the 'code' parameter: 'Promo code to validate and apply.' The tool description adds no new information about the parameter beyond the redundant 'Args: code', so it meets the baseline without improvement.

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 verb 'Validate and apply a promo code to the cart', distinguishing it from sibling tools like 'remove_promo' (which removes a promo) and 'add_to_cart' (which adds items). The resource ('cart') and action are specific and unambiguous.

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?

No guidance is provided on when to use this tool versus alternatives (e.g., 'remove_promo' to remove a promo, or 'cart_update' for other modifications). The description does not mention prerequisites (e.g., cart must exist) or conditions for successful application.

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