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Update cart line

cart_update
Idempotent

Change a cart line by its index (from view_cart): set qty (0 removes), or switch size/fabric. Args: index, qty?, size?, fabric?.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
qtyNoNew quantity; 0 removes the line.
sizeNoNew size for this line (must be available for the product).
indexYesZero-based cart line index from view_cart.
fabricNoNew fabric for this line (must be available for the product).

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
errorNoPresent instead of a cart when the call failed (bad index, unavailable size, ...).
itemsNoCart lines in index order — this index is what set_cart_quantity/cart_update/cart_remove take.
currencyNo
subtotalNoSum of price x qty over all lines, USD.
checkout_urlNoCart page URL for the human.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changed
    • addedInput schema / properties / fabric / description
      Added value: +"New fabric for this line (must be available for the product)."
    • addedInput schema / properties / index / description
      Added value: +"Zero-based cart line index from view_cart."
    • addedInput schema / properties / qty / description
      Added value: +"New quantity; 0 removes the line."
    • addedInput schema / properties / size / description
      Added value: +"New size for this line (must be available for the product)."
    • changedOutput schema / (root)
      Previous value: -nullNew value: +{
+  "properties": {
+    "checkout_url": {
+      "description": "Cart page URL for the human.",
+      "type": "string"
+    },
+    "currency": {
+      "type": "string"
+    },
+    "error": {
+      "description": "Present instead of a cart when the call failed (bad index, unavailable size, ...).",
+      "type": "string"
+    },
+    "items": {
+      "description": "Cart lines in index order — this index is what set_cart_quantity/cart_update/cart_remove take.",
+      "items": {
+        "properties": {
+          "code": {
+            "type": "string"
+          },
+          "currency": {
+            "type": "string"
+          },
+          "custom": {
+            "description": "True when the line is a user-generated custom print.",
+            "type": "boolean"
+          },
+          "fabric": {
+            "type": "string"
+          },
+          "good_id": {
+            "type": "integer"
+          },
+          "price": {
+            "type": "number"
+          },
+          "product": {
+            "type": "string"
+          },
+          "qty": {
+            "type": "integer"
+          },
+          "size": {
+            "type": "string"
+          },
+          "title": {
+            "type": "string"
+          }
+        },
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "subtotal": {
+      "description": "Sum of price x qty over all lines, USD.",
+      "type": "number"
+    }
+  },
+  "type": "object"
+}
  2. First observed

TDQS

A4.2/5.0
Behavior4/5

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

The description adds behavioral details beyond annotations, such as setting qty to 0 to remove the line. Although annotations mark destructiveHint=false, the description clarifies that removal is possible. It also conveys that index comes from view_cart.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is highly concise: one sentence with a colon and then the parameter list. Every word is necessary and no repetition.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The description covers the main use cases (changing qty, size, fabric, removal). An output schema exists, so return values are documented separately. It could mention availability constraints for size and fabric, but those are in the schema.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

All four parameters are described in the schema (100% coverage), so the description adds no new semantic meaning beyond listing the parameters. The note about qty=0 is already in the schema description.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the verb 'Change' and the resource 'a cart line', specifying identification by index from view_cart. It distinguishes itself from siblings like add_to_cart, cart_remove, and set_cart_quantity by covering quantity, size, and fabric updates in one tool.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides context ('from view_cart') and mentions that setting qty to 0 removes the line, implying when to use removal. However, it does not explicitly state when not to use this tool or point to alternatives for adding or removing entirely.

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

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