diff --git a/docs/specification/product-addons.md b/docs/specification/product-addons.md
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+<!--
+   Copyright 2026 UCP Authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+-->
+
+# Product Addons Extension
+
+## Overview
+
+The Product Addons extension enables shoppers to select and change
+product addons (e.g., Warranty, Engraving, Gift Wrapping) directly within
+cart and checkout sessions. Addons are optional purchasable extras associated
+with a product — unlike variant-defining options (Size, Color) which are
+handled at the catalog level, addons represent supplementary choices that
+affect pricing without changing the base product variant.
+
+Without this extension, businesses must either inflate their product catalogs
+with pre-built variants for every addon permutation, or handle addon
+selection entirely outside the checkout flow.
+
+**Key features:**
+
+- Select product addons when adding items to cart or checkout
+- Change addons during the session (e.g., upgrade warranty from 1-year to 2-year)
+- Each addon choice is an item with its own ID, title, and totals
+- Business resolves line item pricing from product + selected addons
+
+**Dependencies:**
+
+- Cart Capability or Checkout Capability
+
+## Discovery
+
+Businesses advertise product addons support in their profile. The capability
+can extend cart, checkout, or both:
+
+```json
+{
+  "ucp": {
+    "version": "{{ ucp_version }}",
+    "capabilities": {
+      "dev.ucp.shopping.product_addons": [
+        {
+          "version": "{{ ucp_version }}",
+          "extends": ["dev.ucp.shopping.cart", "dev.ucp.shopping.checkout"],
+          "spec": "https://ucp.dev/{{ ucp_version }}/specification/product-addons",
+          "schema": "https://ucp.dev/{{ ucp_version }}/schemas/shopping/product_addons.json"
+        }
+      ]
+    }
+  }
+}
+```
+
+## Schema
+
+When this capability is active, line items in cart and/or checkout are extended
+with an `addons` array on the `item` object. Each addon follows the same
+selection pattern as fulfillment groups: the platform sends
+`selected_choice_id`, and the server responds with the full set of available
+addon `choices` including totals.
+
+### Item Addon
+
+{{ extension_schema_fields('product_addons.json#/$defs/item_addon', 'product-addons') }}
+
+### Addon Choice
+
+{{ extension_schema_fields('product_addons.json#/$defs/addon_choice', 'product-addons') }}
+
+## How It Works
+
+### Addon discovery
+
+When `item.id` refers to a product that has available addons, the server
+returns `item.addons` in the response with all available addons and their
+choices. The platform can create a checkout in three ways:
+
+1. **Product ID only** — send `item.id` with no `addons`. The server
+   selects the defaults and returns `addons` with pre-selected choices.
+   The platform can then render addon selectors for the shopper.
+2. **Product ID + partial addons** — send `item.id` with some addons
+   specified (e.g., only Warranty). The server uses defaults for
+   unspecified addons.
+3. **Product ID + full addons** — send `item.id` with all addons
+   specified. The server resolves the exact pricing.
+
+When `item.addons` is absent and the product has no available addons,
+the item behaves as a standard line item.
+
+### Selection pattern
+
+Each addon (Warranty, Engraving, etc.) has:
+
+- `name` — the addon name (e.g., `"Warranty"`)
+- `selected_choice_id` — the ID of the chosen addon choice, sent by the
+  platform. May be omitted on create to let the server select the default.
+- `choices` — all addon choices with pricing, returned by the server
+  (response-only)
+
+Each addon choice has `id`, `title`, and optional `image_url` — plus
+`default` and `totals` representing the cost of that specific choice.
+
+This mirrors the fulfillment group pattern where `selected_option_id`
+chooses among `options[]`.
+
+### Addon choices
+
+Each entry in the `choices` array has:
+
+| Field | Type | Description |
+| :---- | :--- | :---------- |
+| `id` | string | Addon choice identifier, used as `selected_choice_id` |
+| `title` | string | Display title (e.g., "2-Year Extended Warranty") |
+| `image_url` | string | Optional addon image URI |
+| `default` | boolean | Whether this is the default selection when the platform does not specify one |
+| `totals` | array | Totals for this specific addon choice |
+
+The `totals` array represents the cost of the addon choice itself (e.g.,
+a 2-Year Warranty at $300). The platform can use these to display addon
+pricing and compute differences between choices.
+
+### Changing addons
+
+To change an addon, the platform sends an update with the new
+`selected_choice_id`. The server recalculates pricing and returns
+updated totals.
+
+For example, upgrading warranty from 1-year to 2-year:
+
+1. Platform sends update with `selected_choice_id: "warranty_2yr"` for
+   the Warranty addon
+2. Server recalculates the line item price with the new warranty
+3. Response includes updated `item.price`, `item.title`, and refreshed
+   addon `choices` with pricing
+
+### Error handling
+
+When a selected addon choice is invalid or unavailable, the server
+communicates this via `messages[]`:
+
+| Code | Description |
+| :--- | :---------- |
+| `invalid_addon` | The selected choice ID is not recognized for this addon |
+| `addon_unavailable` | The selected addon choice is not available for this product |
+
+## Examples
+
+### Create checkout with product ID only
+
+The platform sends just the product ID — no addons. The server recognizes
+the product has available addons, selects defaults, and returns all addons
+with pre-selected choices. Each addon choice has its own title and totals showing the addon cost.
+
+=== "Request"
+
+    ```json
+    {
+      "line_items": [
+        {
+          "item": {
+            "id": "prod_smartphone_x"
+          },
+          "quantity": 1
+        }
+      ]
+    }
+    ```
+
+=== "Response"
+
+    ```json
+    {
+      "id": "chk_abc123",
+      "status": "incomplete",
+      "currency": "USD",
+      "line_items": [
+        {
+          "id": "li_1",
+          "item": {
+            "id": "prod_smartphone_x",
+            "title": "Smartphone X - 128 GB, Midnight",
+            "price": 89900,
+            "image_url": "https://merchant.example.com/smartphone-x-midnight.jpg",
+            "addons": [
+              {
+                "name": "Warranty",
+                "selected_choice_id": "warranty_none",
+                "choices": [
+                  {
+                    "id": "warranty_none",
+                    "title": "No Warranty",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "warranty_1yr",
+                    "title": "1-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 10000}, {"type": "total", "amount": 10000}]
+                  },
+                  {
+                    "id": "warranty_2yr",
+                    "title": "2-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 30000}, {"type": "total", "amount": 30000}]
+                  }
+                ]
+              },
+              {
+                "name": "Engraving",
+                "selected_choice_id": "engraving_none",
+                "choices": [
+                  {
+                    "id": "engraving_none",
+                    "title": "No Engraving",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "engraving_text",
+                    "title": "Custom Text Engraving",
+                    "image_url": "https://merchant.example.com/engraving-text.jpg",
+                    "totals": [{"type": "subtotal", "amount": 4000}, {"type": "total", "amount": 4000}]
+                  },
+                  {
+                    "id": "engraving_emoji",
+                    "title": "Emoji Engraving",
+                    "image_url": "https://merchant.example.com/engraving-emoji.jpg",
+                    "totals": [{"type": "subtotal", "amount": 2000}, {"type": "total", "amount": 2000}]
+                  }
+                ]
+              },
+              {
+                "name": "Gift Wrapping",
+                "selected_choice_id": "gift_none",
+                "choices": [
+                  {
+                    "id": "gift_none",
+                    "title": "No Gift Wrapping",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "gift_standard",
+                    "title": "Standard Gift Box",
+                    "image_url": "https://merchant.example.com/gift-box-standard.jpg",
+                    "totals": [{"type": "subtotal", "amount": 4900}, {"type": "total", "amount": 4900}]
+                  },
+                  {
+                    "id": "gift_premium",
+                    "title": "Premium Gift Box",
+                    "image_url": "https://merchant.example.com/gift-box-premium.jpg",
+                    "totals": [{"type": "subtotal", "amount": 9900}, {"type": "total", "amount": 9900}]
+                  }
+                ]
+              }
+            ]
+          },
+          "quantity": 1,
+          "totals": [
+            {"type": "subtotal", "amount": 89900},
+            {"type": "total", "amount": 89900}
+          ]
+        }
+      ],
+      "totals": [
+        {"type": "subtotal", "display_text": "Subtotal", "amount": 89900},
+        {"type": "total", "display_text": "Total", "amount": 89900}
+      ]
+    }
+    ```
+
+### Create checkout with selected addons
+
+A shopper adds a smartphone with a warranty and engraving pre-selected.
+
+=== "Request"
+
+    ```json
+    {
+      "line_items": [
+        {
+          "item": {
+            "id": "prod_smartphone_x",
+            "addons": [
+              {"name": "Warranty", "selected_choice_id": "warranty_1yr"},
+              {"name": "Engraving", "selected_choice_id": "engraving_text"},
+              {"name": "Gift Wrapping", "selected_choice_id": "gift_none"}
+            ]
+          },
+          "quantity": 1
+        }
+      ]
+    }
+    ```
+
+=== "Response"
+
+    ```json
+    {
+      "id": "chk_abc123",
+      "status": "incomplete",
+      "currency": "USD",
+      "line_items": [
+        {
+          "id": "li_1",
+          "item": {
+            "id": "prod_smartphone_x",
+            "title": "Smartphone X - 128 GB, Midnight",
+            "price": 103900,
+            "image_url": "https://merchant.example.com/smartphone-x-midnight.jpg",
+            "addons": [
+              {
+                "name": "Warranty",
+                "selected_choice_id": "warranty_1yr",
+                "choices": [
+                  {
+                    "id": "warranty_none",
+                    "title": "No Warranty",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "warranty_1yr",
+                    "title": "1-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 10000}, {"type": "total", "amount": 10000}]
+                  },
+                  {
+                    "id": "warranty_2yr",
+                    "title": "2-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 30000}, {"type": "total", "amount": 30000}]
+                  }
+                ]
+              },
+              {
+                "name": "Engraving",
+                "selected_choice_id": "engraving_text",
+                "choices": [
+                  {
+                    "id": "engraving_none",
+                    "title": "No Engraving",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "engraving_text",
+                    "title": "Custom Text Engraving",
+                    "totals": [{"type": "subtotal", "amount": 4000}, {"type": "total", "amount": 4000}]
+                  },
+                  {
+                    "id": "engraving_emoji",
+                    "title": "Emoji Engraving",
+                    "totals": [{"type": "subtotal", "amount": 2000}, {"type": "total", "amount": 2000}]
+                  }
+                ]
+              },
+              {
+                "name": "Gift Wrapping",
+                "selected_choice_id": "gift_none",
+                "choices": [
+                  {
+                    "id": "gift_none",
+                    "title": "No Gift Wrapping",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "gift_standard",
+                    "title": "Standard Gift Box",
+                    "totals": [{"type": "subtotal", "amount": 4900}, {"type": "total", "amount": 4900}]
+                  },
+                  {
+                    "id": "gift_premium",
+                    "title": "Premium Gift Box",
+                    "totals": [{"type": "subtotal", "amount": 9900}, {"type": "total", "amount": 9900}]
+                  }
+                ]
+              }
+            ]
+          },
+          "quantity": 1,
+          "totals": [
+            {"type": "subtotal", "amount": 103900},
+            {"type": "total", "amount": 103900}
+          ]
+        }
+      ],
+      "totals": [
+        {"type": "subtotal", "display_text": "Subtotal", "amount": 103900},
+        {"type": "total", "display_text": "Total", "amount": 103900}
+      ]
+    }
+    ```
+
+### Upgrade warranty during checkout
+
+The shopper upgrades from 1-Year to 2-Year warranty. The platform sends
+an update with the new `selected_choice_id` — only the warranty selection
+changes. The server recalculates pricing.
+
+=== "Request"
+
+    ```json
+    {
+      "line_items": [
+        {
+          "id": "li_1",
+          "item": {
+            "id": "prod_smartphone_x",
+            "addons": [
+              {"name": "Warranty", "selected_choice_id": "warranty_2yr"},
+              {"name": "Engraving", "selected_choice_id": "engraving_text"},
+              {"name": "Gift Wrapping", "selected_choice_id": "gift_none"}
+            ]
+          },
+          "quantity": 1
+        }
+      ]
+    }
+    ```
+
+=== "Response"
+
+    ```json
+    {
+      "id": "chk_abc123",
+      "status": "incomplete",
+      "currency": "USD",
+      "line_items": [
+        {
+          "id": "li_1",
+          "item": {
+            "id": "prod_smartphone_x",
+            "title": "Smartphone X - 128 GB, Midnight",
+            "price": 123900,
+            "image_url": "https://merchant.example.com/smartphone-x-midnight.jpg",
+            "addons": [
+              {
+                "name": "Warranty",
+                "selected_choice_id": "warranty_2yr",
+                "choices": [
+                  {
+                    "id": "warranty_none",
+                    "title": "No Warranty",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "warranty_1yr",
+                    "title": "1-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 10000}, {"type": "total", "amount": 10000}]
+                  },
+                  {
+                    "id": "warranty_2yr",
+                    "title": "2-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 30000}, {"type": "total", "amount": 30000}]
+                  }
+                ]
+              },
+              {
+                "name": "Engraving",
+                "selected_choice_id": "engraving_text",
+                "choices": [
+                  {
+                    "id": "engraving_none",
+                    "title": "No Engraving",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "engraving_text",
+                    "title": "Custom Text Engraving",
+                    "totals": [{"type": "subtotal", "amount": 4000}, {"type": "total", "amount": 4000}]
+                  },
+                  {
+                    "id": "engraving_emoji",
+                    "title": "Emoji Engraving",
+                    "totals": [{"type": "subtotal", "amount": 2000}, {"type": "total", "amount": 2000}]
+                  }
+                ]
+              },
+              {
+                "name": "Gift Wrapping",
+                "selected_choice_id": "gift_none",
+                "choices": [
+                  {
+                    "id": "gift_none",
+                    "title": "No Gift Wrapping",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "gift_standard",
+                    "title": "Standard Gift Box",
+                    "totals": [{"type": "subtotal", "amount": 4900}, {"type": "total", "amount": 4900}]
+                  },
+                  {
+                    "id": "gift_premium",
+                    "title": "Premium Gift Box",
+                    "totals": [{"type": "subtotal", "amount": 9900}, {"type": "total", "amount": 9900}]
+                  }
+                ]
+              }
+            ]
+          },
+          "quantity": 1,
+          "totals": [
+            {"type": "subtotal", "amount": 123900},
+            {"type": "total", "amount": 123900}
+          ]
+        }
+      ],
+      "totals": [
+        {"type": "subtotal", "display_text": "Subtotal", "amount": 123900},
+        {"type": "total", "display_text": "Total", "amount": 123900}
+      ]
+    }
+    ```
+
+### Mixed items — with and without addons
+
+A checkout containing a smartphone with addons and a phone case with no
+addons (standard line item). The two patterns coexist within the same
+session.
+
+=== "Request"
+
+    ```json
+    {
+      "line_items": [
+        {
+          "item": {
+            "id": "prod_smartphone_x",
+            "addons": [
+              {"name": "Warranty", "selected_choice_id": "warranty_1yr"},
+              {"name": "Engraving", "selected_choice_id": "engraving_none"},
+              {"name": "Gift Wrapping", "selected_choice_id": "gift_standard"}
+            ]
+          },
+          "quantity": 1
+        },
+        {
+          "item": {
+            "id": "sku_case_midnight_clear"
+          },
+          "quantity": 1
+        }
+      ]
+    }
+    ```
+
+=== "Response"
+
+    ```json
+    {
+      "id": "chk_def456",
+      "status": "incomplete",
+      "currency": "USD",
+      "line_items": [
+        {
+          "id": "li_1",
+          "item": {
+            "id": "prod_smartphone_x",
+            "title": "Smartphone X - 128 GB, Midnight",
+            "price": 104800,
+            "image_url": "https://merchant.example.com/smartphone-x-midnight.jpg",
+            "addons": [
+              {
+                "name": "Warranty",
+                "selected_choice_id": "warranty_1yr",
+                "choices": [
+                  {
+                    "id": "warranty_none",
+                    "title": "No Warranty",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "warranty_1yr",
+                    "title": "1-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 10000}, {"type": "total", "amount": 10000}]
+                  },
+                  {
+                    "id": "warranty_2yr",
+                    "title": "2-Year Extended Warranty",
+                    "totals": [{"type": "subtotal", "amount": 30000}, {"type": "total", "amount": 30000}]
+                  }
+                ]
+              },
+              {
+                "name": "Engraving",
+                "selected_choice_id": "engraving_none",
+                "choices": [
+                  {
+                    "id": "engraving_none",
+                    "title": "No Engraving",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "engraving_text",
+                    "title": "Custom Text Engraving",
+                    "totals": [{"type": "subtotal", "amount": 4000}, {"type": "total", "amount": 4000}]
+                  },
+                  {
+                    "id": "engraving_emoji",
+                    "title": "Emoji Engraving",
+                    "totals": [{"type": "subtotal", "amount": 2000}, {"type": "total", "amount": 2000}]
+                  }
+                ]
+              },
+              {
+                "name": "Gift Wrapping",
+                "selected_choice_id": "gift_standard",
+                "choices": [
+                  {
+                    "id": "gift_none",
+                    "title": "No Gift Wrapping",
+                    "default": true,
+                    "totals": [{"type": "subtotal", "amount": 0}, {"type": "total", "amount": 0}]
+                  },
+                  {
+                    "id": "gift_standard",
+                    "title": "Standard Gift Box",
+                    "image_url": "https://merchant.example.com/gift-box-standard.jpg",
+                    "totals": [{"type": "subtotal", "amount": 4900}, {"type": "total", "amount": 4900}]
+                  },
+                  {
+                    "id": "gift_premium",
+                    "title": "Premium Gift Box",
+                    "image_url": "https://merchant.example.com/gift-box-premium.jpg",
+                    "totals": [{"type": "subtotal", "amount": 9900}, {"type": "total", "amount": 9900}]
+                  }
+                ]
+              }
+            ]
+          },
+          "quantity": 1,
+          "totals": [
+            {"type": "subtotal", "amount": 104800},
+            {"type": "total", "amount": 104800}
+          ]
+        },
+        {
+          "id": "li_2",
+          "item": {
+            "id": "sku_case_midnight_clear",
+            "title": "Clear Case - Midnight",
+            "price": 4900
+          },
+          "quantity": 1,
+          "totals": [
+            {"type": "subtotal", "amount": 4900},
+            {"type": "total", "amount": 4900}
+          ]
+        }
+      ],
+      "totals": [
+        {"type": "subtotal", "display_text": "Subtotal", "amount": 109700},
+        {"type": "total", "display_text": "Total", "amount": 109700}
+      ]
+    }
+    ```
diff --git a/mkdocs.yml b/mkdocs.yml
index 36cfef1f..04271a9a 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -53,6 +53,7 @@ nav:
           - Discounts Extension: specification/discount.md
           - Fulfillment Extension: specification/fulfillment.md
           - Loyalty Extension: specification/loyalty.md
+          - Product Addons Extension: specification/product-addons.md
           - Split Payments Extension: specification/split-payments.md
       - Cart Capability:
           - Overview: specification/cart.md
@@ -323,6 +324,10 @@ plugins:
               memberships, tiers, benefits, and reward earning forecasts while
               managing provisional eligibility claims and enforcing
               data-minimization privacy rules.
+          - specification/product-addons.md: >-
+              Product Addons Extension, defining addon
+              discovery, choice selection patterns, pricing resolution,
+              and mid-checkout addon changes.
         Cart Capability:
           - specification/cart.md: >-
               Pre-purchase Cart Capability, detailing item collection, state
diff --git a/source/schemas/shopping/product_addons.json b/source/schemas/shopping/product_addons.json
new file mode 100644
index 00000000..67863541
--- /dev/null
+++ b/source/schemas/shopping/product_addons.json
@@ -0,0 +1,161 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/product_addons.json",
+  "name": "dev.ucp.shopping.product_addons",
+  "title": "Product Addons Extension",
+  "description": "Extends Cart and Checkout with product addon selection on line items, allowing shoppers to select and change product addons (e.g., Warranty, Engraving, Gift Wrapping) at checkout time.",
+  "$defs": {
+    "addon_choice": {
+      "title": "Addon Choice",
+      "description": "A purchasable addon choice with availability and line item totals showing the cost if this addon were selected.",
+      "type": "object",
+      "required": ["id", "title"],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Addon item identifier. Used as selected_choice_id to select this choice."
+        },
+        "title": {
+          "type": "string",
+          "description": "Display title (e.g., '2-Year Extended Warranty', 'Custom Text Engraving')."
+        },
+        "image_url": {
+          "type": "string",
+          "format": "uri",
+          "description": "Addon image URI."
+        },
+        "default": {
+          "type": "boolean",
+          "description": "Whether this is the default selection for the addon when the platform does not specify a selected_choice_id."
+        },
+        "totals": {
+          "type": "array",
+          "items": {
+            "$ref": "types/total.json"
+          },
+          "description": "Totals for this specific addon choice. Enables the platform to display the addon cost and compute price differences between choices."
+        }
+      }
+    },
+    "item_addon": {
+      "title": "Item Addon",
+      "description": "A product addon on a line item with the current selection and available choices.",
+      "type": "object",
+      "required": ["name", "selected_choice_id"],
+      "properties": {
+        "name": {
+          "type": "string",
+          "description": "Addon name (e.g., 'Warranty', 'Engraving', 'Gift Wrapping')."
+        },
+        "selected_choice_id": {
+          "type": "string",
+          "description": "ID of the selected addon choice. On create, may be omitted to let the server select the default. Always present in responses.",
+          "ucp_request": {
+            "create": "optional",
+            "update": "optional"
+          }
+        },
+        "choices": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/addon_choice"
+          },
+          "minItems": 1,
+          "description": "Available addon choices with pricing and availability.",
+          "ucp_request": "omit"
+        }
+      }
+    },
+    "item": {
+      "title": "Item with Product Addons",
+      "description": "Item extended with product addon selections.",
+      "allOf": [
+        {
+          "$ref": "types/item.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "addons": {
+              "type": "array",
+              "items": {
+                "$ref": "#/$defs/item_addon"
+              },
+              "description": "Product addon selections for this item. When present, the server resolves pricing based on the product and selected addons. May be omitted on create — the server returns addons with default selections in the response.",
+              "ucp_request": {
+                "create": "optional",
+                "update": "optional"
+              }
+            }
+          }
+        }
+      ]
+    },
+    "line_item": {
+      "title": "Line Item with Product Addons",
+      "description": "Line item with product addon support on the item.",
+      "allOf": [
+        {
+          "$ref": "types/line_item.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "item": {
+              "$ref": "#/$defs/item"
+            }
+          }
+        }
+      ]
+    },
+    "dev.ucp.shopping.checkout": {
+      "title": "Checkout with Product Addons",
+      "description": "Checkout extended with product addon selection on line items.",
+      "allOf": [
+        {
+          "$ref": "checkout.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "line_items": {
+              "type": "array",
+              "items": {
+                "$ref": "#/$defs/line_item"
+              },
+              "ucp_request": {
+                "create": "required",
+                "update": "required",
+                "complete": "omit"
+              }
+            }
+          }
+        }
+      ]
+    },
+    "dev.ucp.shopping.cart": {
+      "title": "Cart with Product Addons",
+      "description": "Cart extended with product addon selection on line items.",
+      "allOf": [
+        {
+          "$ref": "cart.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "line_items": {
+              "type": "array",
+              "items": {
+                "$ref": "#/$defs/line_item"
+              },
+              "ucp_request": {
+                "create": "required",
+                "update": "required"
+              }
+            }
+          }
+        }
+      ]
+    }
+  }
+}