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

Annotations already flag the operation as destructive and idempotent, but the description adds real behavioral context: it returns the updated cart and checkout_url, no payment or order is placed, checkout happens on shop.nevini.wine, and age verification may occur on delivery. This goes well beyond what the annotations provide and is consistent with them.

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 key operational sentence is front-loaded and efficient, but the description then repeats the checkout-order idea across two separate sentences and includes multiple sentences of alcohol age policy. That policy is relevant for this store, but the repetition and length keep it from being tight.

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?

With an output schema present and annotations covering safety/idempotency, the description supplies the remaining context an agent needs: what the tool returns, that checkout_url is where checkout happens, and the age restrictions for this alcohol seller. Nothing essential to calling it correctly is missing.

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 schema itself documents all three parameters, including quantity's 0-24 range and the cart_id key requirement, while the description only restates that quantity 0 removes a line. It adds little meaning beyond the schema and does not explain the language parameter or cart_id format, so baseline 3 is appropriate at 67% schema coverage.

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 first sentence names a specific verb and resource: sets quantities of existing cart lines by line_id, with quantity 0 removing a line. This clearly distinguishes it from add_to_cart and create_cart, which handle adding and creating rather than modifying existing lines.

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?

Usage is implied by 'existing cart lines' and the removal behavior for quantity 0, and the description clarifies that no order is placed here. However, it never explicitly names alternatives such as add_to_cart for new lines or create_cart for a new cart, so the agent must infer when to choose this tool over its siblings.

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