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

With no annotations, the description carries the full burden. It usefully discloses a key behavioral fact: 'This does not charge the buyer,' which prevents confusion with checkout/payment tools. However, it does not explain failure behavior, idempotency, or exactly how quantity is applied (incremented vs. set), leaving behavioral transparency incomplete.

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 short sentences with no filler. Each sentence has a distinct job: state the action, provide the usage trigger, and clarify a critical non-charge behavior. The core purpose is front-loaded, making it easy to scan.

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?

There is no output schema, no annotations, and zero parameter descriptions in the schema. The description does not explain site_slug's role, the default quantity behavior, or what the tool returns. For a mutation tool with required fields and no structured safety hints, the definition is not complete enough for an agent to confidently invoke it.

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 description must compensate. It loosely connects 'product variant' to variant_id and 'existing cart' to cart_id, but it never explains site_slug or how quantity behaves when omitted. The agent is left to infer the meaning of required parameters from context, which is insufficient for a 4-parameter tool.

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 states a specific verb and resource: 'Add another selected product variant to an existing Agent Checkout cart.' It clearly distinguishes this from create_cart and create_checkout by emphasizing 'existing' cart and 'another' item. The additional note that it does not charge the buyer further clarifies the tool's boundary.

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?

The description provides an explicit usage condition: 'Use when the user explicitly wants another item or quantity added to an existing cart.' It gives clear context for when to invoke the tool, though it does not name alternatives or explicit when-not-to-use conditions beyond what is implied by 'existing cart.'

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