update_cart_item

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

With no annotations, the description carries the full burden. It discloses two valuable behavioral facts (declarative 'exact' set semantics rather than increment, and that zero removes the line) plus the no-charge reassurance. It omits what happens when product_id is not already in the cart, any auth requirements, and whether the operation is idempotent.

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 short sentences, no filler, with the core action and the critical zero-quantity edge case front-loaded. Every sentence 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?

An output schema exists, so return values need no explanation, and no annotations means the safety profile is largely self-evident for a non-charging cart edit. The remaining gap is the undocumented product_id semantics, which the description does not close.

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 0%, so the schema contributes nothing beyond types and required-ness. The description compensates for quantity by defining the zero-value edge case, but product_id is left entirely unexplained (must it already be in the cart? is it a catalog id?). Partial compensation only.

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 and resource ('Set the exact quantity of a cart item'), and the word 'exact' implicitly contrasts this with the additive sibling add_to_cart. An agent can route between the two without opening either schema.

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 rather than stated: 'quantity=0 removes it' tells the agent this tool doubles as a removal path, but there is no explicit when-to-use vs add_to_cart, clear_cart, or view_cart. Adequate for a simple two-parameter cart mutation, but no alternatives or prerequisites are named.

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