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

Adds one genuinely valuable constraint not in the annotations: "One merchant per cart." The annotations already declare this is a non-read-only, non-destructive, open-world mutation, so the safety profile is covered. However, the description says nothing about what happens when the cart already contains items from another merchant (error vs. replace) or whether repeated adds merge quantities — the practical failure modes for this tool.

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?

Front-loaded with the core action and then organized around the three usage shapes, so it is easy to scan. The parenthetical "(put a whole request's items in one call)" restates the preceding clause and could be dropped without loss.

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 description covers the main invocation shapes plus the cross-merchant constraint. What it leaves out — conflict behavior with a non-empty cart and the unmentioned `language` param — are the remaining meaningful holes for a mutation tool with five optional parameters.

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 already 100%, but the description earns credit above baseline by explaining the mutually exclusive parameter paths (items vs. productId/quantity/optionValueIds) and by translating optionValueIds into domain terms (addon/size value ids). The `language` parameter is never mentioned in the description, which is the one gap.

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?

Opens with a specific verb and resource ("Add one or more products to the cart") plus an explicit scope qualifier ("in a single call"), which immediately separates it from update_cart_item, remove_cart_item, and clear_cart. An agent can select it without opening the 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 routing rules for each shape: `quantity` for multiple of the SAME item, `items` for DIFFERENT products, direct params for a single product. It also names where product ids come from (search_merchant_products/get_product_details), which is the real prerequisite an agent needs.

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