update_cart_item

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

Annotations already declare readOnlyHint=false and destructiveHint=false; the description adds the non-obvious behavioral rule that quantity=0 deletes the line, which the schema cannot express. It does not describe error behavior (e.g. invalid row_id) or whether the result is reversible, but with annotations covering the safety profile this is solid.

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?

A single front-loaded sentence with no filler; the operation comes first and the two operational details are parenthetically appended. 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?

For a two-param cart mutation with annotations and an output schema (so return values need no explanation), the description covers the essential gotchas. Minor omissions (failure modes when row_id is stale) keep it from being fully complete.

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 0%, so the description carries the burden, and it does so for both params: row_id is defined as coming from view_cart output and quantity gets a special sentinel meaning (0 = removal). This is meaningful semantics beyond the bare 'string'/'integer' schema types.

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+resource ('Change the quantity of a cart line') and distinguishes itself from siblings like add_to_cart and clear_cart by naming the exact mutation it performs. An agent can identify the operation 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?

Points the agent to view_cart as the source of row_id and reports that quantity=0 removes the line, which is the key routing condition (no separate remove tool needed). It does not explicitly contrast itself with add_to_cart/clear_cart, so it stops short of full when/when-not guidance.

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