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

Annotations already declare readOnlyHint=false and idempotentHint=false, but the description adds two things they do not: the required OAuth scope (cart:write) and the state-change consequence ('Changes buyer or order state'). It reinforces the non-idempotent warning with 'do not retry blindly', though it does not say what exactly is mutated in the basket.

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 dense sentences with zero filler; the core action and increment rule come first, followed by the retry/order exclusion and then auth/state notes. Every clause 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 the input schema fully documents both parameters. The description covers the remaining agent-facing needs for a non-idempotent mutation: scope, state-change warning, and misuse exclusions.

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 are fully documented in the schema, including the increment-vs-final-quantity distinction and the product ID vs SKU warning. The description's 'Quantity is an increment' merely repeats the schema, adding no new parameter meaning, so the baseline 3 is correct.

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), resource (product quantity), and target (existing basket), and the increment semantics ('Quantity is an increment') cleanly separates it from update_cart_item, which sets a final quantity. An agent can distinguish it from add_favorite and prepare_order without opening a 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-not guidance: 'do not retry blindly or use this to create an order', which routes order creation to prepare_order/submit_order. It does not name those siblings directly or state prerequisites beyond the OAuth scope, so it stops just short of a 5.

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