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

Annotations already declare non-idempotent, non-destructive, non-open-world, but the description adds the operative consequence: the quantity is an increment, an uncertain call must be reconciled via get_cart rather than retried, the required OAuth scope cart:write, and that buyer/order state is mutated. That is real value beyond the structured hints.

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?

Four short sentences, front-loaded with the core action, then routing, then failure handling, then boundary. No filler, and the most error-prone constraint (increment, not final quantity) appears early.

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; the description instead covers auth scope, side effects, retry/reconciliation behavior, and the sibling alternative. Nothing an agent needs to call this safely 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?

Schema coverage is 100%, so the baseline is 3, but the description reinforces the counter-intuitive increment semantics of quantity and adds provenance guidance for product_id ('selected with get_product'), which the schema only describes as 'catalog search or a product response.' It slightly exceeds the schema rather than merely restating it.

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 (add) plus resource (cart item) and the exact semantics: 'quantity units of a product selected with get_product to the consenting buyer's basket.' It explicitly distinguishes itself from update_cart_item (increment vs absolute) and get_cart, so an agent can route correctly without opening any 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?

Gives explicit when-to-use routing ('use update_cart_item with an item_id from get_cart for an absolute final quantity'), a failure-mode rule ('Do not retry an uncertain add blindly; read get_cart to reconcile'), and a boundary ('Does not create an order'). Both the alternative tool and the condition selecting it are named.

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