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

Beyond the annotations (destructiveHint=true, readOnlyHint=false), the description adds significant behavioral context: owner binding, full-state PUT semantics for UCP, additive vs. quantity-update vs. removal operations for the standard endpoint, and the special case where quantity 0 removes a line. This aligns with the annotations and gives the agent a clear mental model of side effects.

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 compact and front-loaded: the first sentence states the core purpose and endpoint, the second covers owner restriction, and the third details endpoint-dependent semantics. Every sentence adds value without repetition or filler.

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?

For a complex mutation tool with nested parameters and no output schema, the description covers the essential operational details: owner token requirement, endpoint variations, field mapping, and removal semantics. It does not mention return values or error behavior, which would be useful but is not strictly required given the tool's scope and the presence of annotations.

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 low (25%), but the description compensates by explaining the roles of line_items, update_items, and remove_line_ids, and their mapping to Shopify operations. It also highlights the quantity-0 removal behavior, which is not fully obvious from the schema alone. However, some nested session/context fields remain undocumented in both places.

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 identifies the action (update) and resource (Shopify cart), and distinguishes it from sibling tools like create_cart and get_cart by explicitly mentioning owner-bound access and the required cart_access_token. It also specifies the endpoint type and mapping semantics, leaving no ambiguity about what the tool does.

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 important usage guidance: it states the owner-only constraint and prerequisite token from create_cart, and explains the behavioral differences between UCP and standard Storefront endpoints. However, it does not explicitly contrast with alternatives such as get_cart or when to prefer this over recreate/delete workflows, so a small gap remains.

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