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

The description adds substantial behavioral context beyond annotations. It discloses the return format, identifies lines by skuId (the variant id used when added), and lists specific error reasons (CART_NOT_FOUND, LINE_NOT_FOUND, INVENTORY_INSUFFICIENT, etc.). It also clarifies that quantity=0 is not for removal, which is a subtle behavioral note. No contradictions with annotations.

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?

The description is three sentences, each serving a distinct purpose: action and identification, return format, and alternative usage plus error handling. It is front-loaded with the primary purpose and avoids unnecessary detail, making it highly efficient and well-structured.

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?

Given the tool's simplicity and the absence of an output schema, the description provides complete context: it explains what the tool does, how it identifies lines, what it returns, when to use an alternative, and what errors to expect. This is sufficient for an agent to select and invoke the tool correctly with no gaps.

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?

With 0% schema description coverage, the description compensates by explaining that skuId is the variant id used when the line was added. Quantity is contextually clear as the new quantity, and cartId is self-explanatory in the cart context. However, it does not explicitly describe cartId or the validation constraints beyond what schema provides, so it's not perfect.

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?

The description clearly states the tool's function: 'Change the quantity of an existing line in a droplinked cart.' It specifies the resource (cart line), the action (update quantity), and the identification method (skuId). It distinguishes itself from cart.removeLine by explicitly noting that removal should use that tool, and from cart.addLine by the update vs add nature.

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?

Explicitly provides an alternative: 'Use cart.removeLine to remove a line entirely instead of setting quantity=0.' This gives clear when-to-use and when-not-to-use guidance. Additionally, the structured error reasons inform the agent on how to handle failures, which aids in deciding when to retry or fall back.

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