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

The description carries behavior the annotations do not: setting quantity to 0 deletes the line, a side effect that reads slightly against destructiveHint=false even though a cart line can be re-added. Otherwise, it says nothing about how a bad or stale line_id is handled, and annotations already cover the non-read-only, closed-world scope.

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 filler. The key mechanism (line_id from view_cart) and the edge case (quantity 0 removes) come first.

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 2-parameter mutation with an output schema, the description covers the inputs and the one non-obvious side effect. The only gap is the missing sibling routing, such as when to use remove_cart_item instead of quantity 0.

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 100%, so both parameters, including the minimum: 0 constraint and the zero-removes rule, are already documented. The description restates those ideas without adding new format or validation detail, so baseline 3 applies.

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 (set quantity) on a specific resource (cart line) keyed by line_id, which is unambiguous. It does not distinguish itself from the related siblings add_to_cart or remove_cart_item, even though a quantity-0 call overlaps with remove_cart_item.

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 the workflow by telling the agent to get line_id from view_cart, and it implicitly covers removal via quantity 0. It never says when to prefer this tool over add_to_cart or remove_cart_item, nor what happens on an invalid line_id, so usage guidance is only implied.

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