diff --git a/docs/specification/cart.md b/docs/specification/cart.md
new file mode 100644
index 000000000..d146db965
--- /dev/null
+++ b/docs/specification/cart.md
@@ -0,0 +1,52 @@
+<!--
+   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.
+-->
+
+# Cart Capability
+
+* **Capability Name:** `dev.ucp.shopping.cart`
+* **Version:** `2026-01-11`
+
+## Overview
+
+The Cart capability provides a first-class basket for multi-item shopping before
+checkout. It allows platforms to build, validate, and update carts without
+collecting payment details.
+
+A cart can be converted into a checkout session by sending the cart's
+`line_items` to the Checkout create endpoint.
+
+## Endpoints
+
+### Create Cart
+
+{{ method_fields('create_cart', 'rest.openapi.json', 'cart') }}
+
+### Get Cart
+
+{{ method_fields('get_cart', 'rest.openapi.json', 'cart') }}
+
+### Update Cart
+
+{{ method_fields('update_cart', 'rest.openapi.json', 'cart') }}
+
+## Schema
+
+{{ schema_fields('cart_resp', 'cart') }}
+
+## Related Specs
+
+* [Checkout Capability](checkout.md)
+* [HTTP/REST Binding](checkout-rest.md)
diff --git a/docs/specification/loyalty.md b/docs/specification/loyalty.md
new file mode 100644
index 000000000..1e93c5e06
--- /dev/null
+++ b/docs/specification/loyalty.md
@@ -0,0 +1,93 @@
+<!--
+   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.
+-->
+
+# Loyalty Extension
+
+* **Capability Name:** `dev.ucp.shopping.loyalty`
+* **Version:** `2026-01-11`
+* **Extends:** `dev.ucp.shopping.checkout`
+
+## Overview
+
+The Loyalty extension lets platforms link a shopper's membership and apply
+eligible benefits during checkout. It pairs with Identity Linking for account
+connection and uses standard checkout updates to apply benefits or redeem
+points.
+
+## Discovery
+
+Businesses advertise loyalty support in their discovery profile:
+
+```json
+{
+  "name": "dev.ucp.shopping.loyalty",
+  "version": "2026-01-11",
+  "spec": "https://ucp.dev/specification/loyalty",
+  "schema": "https://ucp.dev/schemas/shopping/loyalty.json",
+  "extends": "dev.ucp.shopping.checkout"
+}
+```
+
+## Account Linking Flow
+
+1.  The platform links the user's account using the Identity Linking capability.
+2.  The business returns a loyalty `member_id` or indicates linking is required.
+
+See [Identity Linking Capability](identity-linking.md) for OAuth requirements.
+
+## Benefit Application Flow
+
+1.  Platform submits `loyalty` details in a checkout update.
+2.  Business applies benefits and returns `benefits_applied` with updated totals.
+
+### Example checkout update
+
+```json
+{
+  "id": "chk_123",
+  "loyalty": {
+    "program_id": "loyalty_prime",
+    "member_id": "member_456",
+    "redemption": { "points": 1200 }
+  }
+}
+```
+
+### Example checkout response
+
+```json
+{
+  "id": "chk_123",
+  "loyalty": {
+    "program_id": "loyalty_prime",
+    "member_id": "member_456",
+    "status": "linked",
+    "tier": "gold",
+    "points_balance": 4200,
+    "benefits_applied": [
+      {
+        "type": "points_redemption",
+        "amount": 1200,
+        "description": "Redeemed 1,200 points"
+      }
+    ]
+  }
+}
+```
+
+## Schema
+
+{{ extension_schema_fields('loyalty.json#/$defs/loyalty_object', 'checkout') }}
diff --git a/docs/specification/order.md b/docs/specification/order.md
index 1f5fdd2f5..a9fa70c07 100644
--- a/docs/specification/order.md
+++ b/docs/specification/order.md
@@ -95,6 +95,11 @@ fulfillment:
 - Include amount when relevant
 - Can happen at any time regardless of fulfillment status
 
+### Post-Order Management Extension
+
+For structured return, exchange, and refund events, see the
+[Post-Order Management Extension](post-order.md).
+
 ## Schema
 
 ### Order
diff --git a/docs/specification/overview.md b/docs/specification/overview.md
index 803784b95..6ce7e931e 100644
--- a/docs/specification/overview.md
+++ b/docs/specification/overview.md
@@ -181,6 +181,9 @@ Extensions can be:
 -   **Official**: `dev.ucp.shopping.fulfillment` extends `dev.ucp.shopping.checkout`
 -   **Vendor**: `com.example.installments` extends `dev.ucp.shopping.checkout`
 
+Common extensions include `dev.ucp.shopping.discount`, `dev.ucp.shopping.fulfillment`,
+`dev.ucp.shopping.loyalty`, and `dev.ucp.shopping.post_order`.
+
 ### Schema Composition
 
 Extensions can add new fields and modify shared structures (e.g., discounts
@@ -976,6 +979,7 @@ UCP defines a set of standard capabilities:
 
 | Capability Name      | ID (URI)                                         | Description                                                                                                  |
 | :------------------- | :----------------------------------------------- | :----------------------------------------------------------------------------------------------------------- |
+| **Cart**             | `{{ ucp_url }}/schemas/shopping/cart.json`             | Provides a first-class basket for multi-item shopping prior to checkout.                                       |
 | **Checkout**         | `{{ ucp_url }}/schemas/shopping/checkout.json`         | Facilitates the creation and management of checkout sessions, including cart management and tax calculation. |
 | **Identity Linking** | - | Enables platforms to obtain authorization via OAuth 2.0 to perform actions on a user's behalf.               |
 | **Order**            | `{{ ucp_url }}/schemas/shopping/order.json`            | Allows businesses to push asynchronous updates about an order's lifecycle (shipping, delivery, returns).      |
@@ -1168,4 +1172,4 @@ Vendors control their own release schedules and versioning strategy.
 | **Payment Service Provider**      | PSP     | The financial infrastructure provider that processes payments, authorizations, and settlements on behalf of the business.               |
 | **Platform**                      | -       | The consumer-facing surface (AI agent, app, website) acting on behalf of the user to discover businesses and facilitate commerce.                              |
 | **Verifiable Digital Credential** | VDC     | An Issuer-signed credential (set of claims) whose authenticity can be verified cryptographically. Used in UCP for secure payment authorizations.              |
-| **Verifiable Presentation**       | VP      | A presentation of one or more VDCs that includes a cryptographic proof of binding, used to prove authorization to a business or PSP.                          |
\ No newline at end of file
+| **Verifiable Presentation**       | VP      | A presentation of one or more VDCs that includes a cryptographic proof of binding, used to prove authorization to a business or PSP.                          |
diff --git a/docs/specification/post-order.md b/docs/specification/post-order.md
new file mode 100644
index 000000000..0516d319e
--- /dev/null
+++ b/docs/specification/post-order.md
@@ -0,0 +1,78 @@
+<!--
+   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.
+-->
+
+# Post-Order Management Extension
+
+* **Capability Name:** `dev.ucp.shopping.post_order`
+* **Version:** `2026-01-11`
+* **Extends:** `dev.ucp.shopping.order`
+
+## Overview
+
+The Post-Order Management extension standardizes how businesses report returns,
+exchanges, and refunds after a checkout completes. These events supplement the
+base `adjustments` and fulfillment logs with explicit, structured post-order
+signals.
+
+## Return Flow
+
+1.  Platform initiates a return with the business (outside the core UCP flow).
+2.  Business appends a return event to the order response.
+
+## Exchange Flow
+
+1.  Platform requests an exchange with desired replacements.
+2.  Business appends an exchange event to the order response.
+
+## Refund Flow
+
+1.  Business issues a refund after return approval or cancellation.
+2.  Business appends a refund event to the order response.
+
+## Example Order Response
+
+```json
+{
+  "id": "order_123",
+  "post_order": {
+    "returns": [
+      {
+        "id": "ret_1",
+        "status": "approved",
+        "occurred_at": "2026-01-11T14:30:00Z",
+        "line_items": [{ "id": "li_1", "quantity": 1 }],
+        "reason_code": "defective",
+        "description": "Defective item"
+      }
+    ],
+    "refunds": [
+      {
+        "id": "ref_1",
+        "status": "completed",
+        "occurred_at": "2026-01-12T10:15:00Z",
+        "amount": 26550,
+        "line_items": [{ "id": "li_1", "quantity": 1 }],
+        "reason_code": "return_completed",
+        "description": "Refund after return"
+      }
+    ]
+  }
+}
+```
+
+## Schema
+
+{{ extension_schema_fields('post_order.json#/$defs/post_order_object', 'order') }}
diff --git a/mkdocs.yml b/mkdocs.yml
index e39f3880f..16d3b13b2 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -30,6 +30,8 @@ nav:
   - Playground: playground.md
   - Specification:
     - Overview: specification/overview.md
+    - Cart Capability:
+      - Overview: specification/cart.md
     - Checkout Capability:
       - Overview: specification/checkout.md
       - HTTP/REST Binding: specification/checkout-rest.md
@@ -40,7 +42,10 @@ nav:
       - Buyer Consent Extension: specification/buyer-consent.md
       - Discounts Extension: specification/discount.md
       - Fulfillment Extension: specification/fulfillment.md
-    - Order Capability: specification/order.md
+      - Loyalty Extension: specification/loyalty.md
+    - Order Capability:
+      - Overview: specification/order.md
+      - Post-Order Extension: specification/post-order.md
     - Identity Linking Capability: specification/identity-linking.md
     - Payment Handlers:
       - Guide: specification/payment-handler-guide.md
@@ -187,6 +192,8 @@ plugins:
           - documentation/roadmap.md
         Specification Overview:
           - specification/overview.md
+        Cart Capability:
+          - specification/cart.md
         Checkout Capability:
           - specification/checkout.md
           - specification/checkout-rest.md
@@ -198,8 +205,10 @@ plugins:
           - specification/discount.md
           - specification/embedded-checkout.md
           - specification/fulfillment.md
+          - specification/loyalty.md
         Order Capability:
           - specification/order.md
+          - specification/post-order.md
         Identity Linking Capability:
           - specification/identity-linking.md
         Payment Handlers:
diff --git a/source/schemas/shopping/cart.json b/source/schemas/shopping/cart.json
new file mode 100644
index 000000000..5ee9ef65d
--- /dev/null
+++ b/source/schemas/shopping/cart.json
@@ -0,0 +1,94 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/cart.json",
+  "name": "dev.ucp.shopping.cart",
+  "version": "2026-01-11",
+  "title": "Cart",
+  "description": "Cart schema for multi-item baskets prior to checkout.",
+  "type": "object",
+  "required": [
+    "ucp",
+    "id",
+    "line_items",
+    "status",
+    "currency",
+    "totals"
+  ],
+  "additionalProperties": true,
+  "properties": {
+    "ucp": {
+      "$ref": "../ucp.json#/$defs/response_cart",
+      "ucp_request": "omit"
+    },
+    "id": {
+      "type": "string",
+      "description": "Unique cart identifier.",
+      "ucp_request": {
+        "create": "omit",
+        "update": "required"
+      }
+    },
+    "line_items": {
+      "type": "array",
+      "items": {
+        "$ref": "types/line_item.json"
+      },
+      "description": "List of line items in the cart."
+    },
+    "buyer": {
+      "$ref": "types/buyer.json",
+      "description": "Representation of the buyer."
+    },
+    "status": {
+      "type": "string",
+      "enum": [
+        "active",
+        "ready_for_checkout",
+        "converted",
+        "abandoned",
+        "expired"
+      ],
+      "description": "Cart lifecycle status.",
+      "ucp_request": "omit"
+    },
+    "currency": {
+      "type": "string",
+      "description": "ISO 4217 currency code."
+    },
+    "totals": {
+      "type": "array",
+      "items": {
+        "$ref": "types/total.json"
+      },
+      "description": "Different cart totals.",
+      "ucp_request": "omit"
+    },
+    "messages": {
+      "type": "array",
+      "items": {
+        "$ref": "types/message.json"
+      },
+      "description": "Messages about cart state.",
+      "ucp_request": "omit"
+    },
+    "links": {
+      "type": "array",
+      "items": {
+        "$ref": "types/link.json"
+      },
+      "description": "Links to policies and terms.",
+      "ucp_request": "omit"
+    },
+    "expires_at": {
+      "type": "string",
+      "format": "date-time",
+      "description": "RFC 3339 expiry timestamp.",
+      "ucp_request": "omit"
+    },
+    "checkout_id": {
+      "type": "string",
+      "description": "Checkout session created from this cart, if converted.",
+      "ucp_request": "omit"
+    }
+  }
+}
diff --git a/source/schemas/shopping/loyalty.json b/source/schemas/shopping/loyalty.json
new file mode 100644
index 000000000..f8eb4270e
--- /dev/null
+++ b/source/schemas/shopping/loyalty.json
@@ -0,0 +1,102 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/loyalty.json",
+  "name": "dev.ucp.shopping.loyalty",
+  "version": "2026-01-11",
+  "title": "Loyalty Extension",
+  "description": "Extends Checkout with loyalty program linkage and benefit application.",
+  "$defs": {
+    "benefit": {
+      "type": "object",
+      "description": "A loyalty benefit applied to the checkout.",
+      "required": ["type", "amount"],
+      "properties": {
+        "type": {
+          "type": "string",
+          "description": "Benefit type (e.g., points_redemption, member_discount, free_shipping)."
+        },
+        "amount": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Benefit value in minor (cents) currency units or points, depending on type."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable description of the applied benefit."
+        }
+      }
+    },
+    "redemption": {
+      "type": "object",
+      "description": "Requested redemption inputs from the platform.",
+      "properties": {
+        "points": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Number of points to redeem.",
+          "ucp_request": "optional"
+        }
+      }
+    },
+    "loyalty_object": {
+      "type": "object",
+      "description": "Loyalty program linkage and benefit application state.",
+      "required": ["program_id"],
+      "properties": {
+        "program_id": {
+          "type": "string",
+          "description": "Identifier for the loyalty program."
+        },
+        "member_id": {
+          "type": "string",
+          "description": "Member identifier for the linked account.",
+          "ucp_request": "optional"
+        },
+        "status": {
+          "type": "string",
+          "enum": ["linked", "unlinked", "pending", "ineligible"],
+          "description": "Loyalty linkage status for this checkout.",
+          "ucp_request": "omit"
+        },
+        "tier": {
+          "type": "string",
+          "description": "Member tier or segment label.",
+          "ucp_request": "omit"
+        },
+        "points_balance": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Member points balance at the time of checkout.",
+          "ucp_request": "omit"
+        },
+        "benefits_applied": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/benefit" },
+          "description": "Benefits applied to the checkout by the business.",
+          "ucp_request": "omit"
+        },
+        "redemption": {
+          "$ref": "#/$defs/redemption",
+          "description": "Requested redemptions submitted by the platform.",
+          "ucp_request": "optional"
+        }
+      }
+    },
+    "checkout": {
+      "title": "Checkout with Loyalty",
+      "description": "Checkout extended with loyalty capability.",
+      "allOf": [
+        { "$ref": "checkout.json" },
+        {
+          "type": "object",
+          "properties": {
+            "loyalty": {
+              "$ref": "#/$defs/loyalty_object",
+              "ucp_request": "optional"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/source/schemas/shopping/post_order.json b/source/schemas/shopping/post_order.json
new file mode 100644
index 000000000..c96ae7b1b
--- /dev/null
+++ b/source/schemas/shopping/post_order.json
@@ -0,0 +1,173 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/post_order.json",
+  "name": "dev.ucp.shopping.post_order",
+  "version": "2026-01-11",
+  "title": "Post-Order Extension",
+  "description": "Extends Order with standardized return, exchange, and refund events.",
+  "$defs": {
+    "line_item_reference": {
+      "type": "object",
+      "required": ["id", "quantity"],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Line item ID reference."
+        },
+        "quantity": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Quantity affected by this event."
+        }
+      }
+    },
+    "return_event": {
+      "type": "object",
+      "description": "Return event for items after purchase.",
+      "required": ["id", "status", "occurred_at"],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Return event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": ["requested", "approved", "rejected", "in_progress", "completed", "canceled"],
+          "description": "Return status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/line_item_reference" },
+          "description": "Line items and quantities included in the return."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable return reason (e.g., defective, wrong_size)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable return reason."
+        }
+      }
+    },
+    "exchange_event": {
+      "type": "object",
+      "description": "Exchange event for swapping items post-purchase.",
+      "required": ["id", "status", "occurred_at"],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Exchange event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": ["requested", "approved", "rejected", "in_progress", "completed", "canceled"],
+          "description": "Exchange status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/line_item_reference" },
+          "description": "Line items and quantities being exchanged."
+        },
+        "replacement_items": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/line_item_reference" },
+          "description": "Replacement line items and quantities."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable exchange reason (e.g., wrong_size, color_change)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable exchange reason."
+        }
+      }
+    },
+    "refund_event": {
+      "type": "object",
+      "description": "Refund event for returned or canceled items.",
+      "required": ["id", "status", "occurred_at", "amount"],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Refund event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": ["pending", "completed", "failed"],
+          "description": "Refund status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this refund occurred."
+        },
+        "amount": {
+          "type": "integer",
+          "description": "Refund amount in minor (cents) currency units."
+        },
+        "line_items": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/line_item_reference" },
+          "description": "Line items and quantities included in the refund."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable refund reason (e.g., return_completed)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable refund reason."
+        }
+      }
+    },
+    "post_order_object": {
+      "type": "object",
+      "description": "Structured post-order events for returns, exchanges, and refunds.",
+      "properties": {
+        "returns": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/return_event" },
+          "description": "Return events for the order."
+        },
+        "exchanges": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/exchange_event" },
+          "description": "Exchange events for the order."
+        },
+        "refunds": {
+          "type": "array",
+          "items": { "$ref": "#/$defs/refund_event" },
+          "description": "Refund events for the order."
+        }
+      }
+    },
+    "order": {
+      "title": "Order with Post-Order Events",
+      "description": "Order extended with post-order management data.",
+      "allOf": [
+        { "$ref": "order.json" },
+        {
+          "type": "object",
+          "properties": {
+            "post_order": {
+              "$ref": "#/$defs/post_order_object",
+              "ucp_request": "omit"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/source/schemas/ucp.json b/source/schemas/ucp.json
index b532a1045..b98d732be 100644
--- a/source/schemas/ucp.json
+++ b/source/schemas/ucp.json
@@ -48,6 +48,21 @@
       }
     },
 
+    "response_cart": {
+      "title": "UCP Cart Response",
+      "description": "UCP metadata for cart responses.",
+      "type": "object",
+      "required": ["version", "capabilities"],
+      "properties": {
+        "version": {"$ref": "#/$defs/version"},
+        "capabilities": {
+          "type": "array",
+          "description": "Active capabilities for this response.",
+          "items": {"$ref": "capability.json#/$defs/response"}
+        }
+      }
+    },
+
     "response_order": {
       "title": "UCP Order Response",
       "description": "UCP metadata for order responses. No payment handlers needed post-purchase.",
diff --git a/source/services/shopping/openapi.json b/source/services/shopping/openapi.json
index 2d7bfacf4..fabf0ead5 100644
--- a/source/services/shopping/openapi.json
+++ b/source/services/shopping/openapi.json
@@ -18,6 +18,188 @@
     }
   ],
   "paths": {
+    "/carts": {
+      "post": {
+        "operationId": "create_cart",
+        "summary": "Create Cart",
+        "description": "Create a cart session. Called when a user begins building a basket.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/idempotency_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": { "$ref": "#/components/schemas/cart" }
+            }
+          }
+        },
+        "responses": {
+          "201": {
+            "description": "Cart created",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/carts/{id}": {
+      "parameters": [
+        { "$ref": "#/components/parameters/cart_id_path" }
+      ],
+      "get": {
+        "operationId": "get_cart",
+        "summary": "Get Cart",
+        "description": "Get the latest state of a cart session.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Cart retrieved",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart"
+                }
+              }
+            }
+          }
+        }
+      },
+      "put": {
+        "operationId": "update_cart",
+        "summary": "Update Cart",
+        "description": "If an optional field is provided in the request body, its value is treated as a complete replacement for the corresponding data. If optional field is omitted, then current cart session is unchanged.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/idempotency_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": { "$ref": "#/components/schemas/cart" }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "Cart updated",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/checkout-sessions": {
       "post": {
         "operationId": "create_checkout",
@@ -400,6 +582,15 @@
   },
   "components": {
     "parameters": {
+      "cart_id_path": {
+        "name": "id",
+        "in": "path",
+        "required": true,
+        "description": "The unique identifier of the cart.",
+        "schema": {
+          "type": "string"
+        }
+      },
       "checkout_session_id_path": {
         "name": "id",
         "in": "path",
@@ -512,6 +703,9 @@
       }
     },
     "schemas": {
+      "cart": {
+        "$ref": "https://ucp.dev/schemas/shopping/cart.json"
+      },
       "checkout": {
         "$ref": "https://ucp.dev/schemas/shopping/checkout.json"
       },
diff --git a/spec/schemas/shopping/cart.create_req.json b/spec/schemas/shopping/cart.create_req.json
new file mode 100644
index 000000000..1317399b3
--- /dev/null
+++ b/spec/schemas/shopping/cart.create_req.json
@@ -0,0 +1,31 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/cart.create_req.json",
+  "name": "dev.ucp.shopping.cart",
+  "version": "2026-01-11",
+  "title": "Cart Create Request",
+  "description": "Cart schema for multi-item baskets prior to checkout.",
+  "type": "object",
+  "required": [
+    "line_items",
+    "currency"
+  ],
+  "additionalProperties": true,
+  "properties": {
+    "line_items": {
+      "type": "array",
+      "items": {
+        "$ref": "types/line_item.create_req.json"
+      },
+      "description": "List of line items in the cart."
+    },
+    "buyer": {
+      "$ref": "types/buyer.json",
+      "description": "Representation of the buyer."
+    },
+    "currency": {
+      "type": "string",
+      "description": "ISO 4217 currency code."
+    }
+  }
+}
diff --git a/spec/schemas/shopping/cart.update_req.json b/spec/schemas/shopping/cart.update_req.json
new file mode 100644
index 000000000..7b08ab25b
--- /dev/null
+++ b/spec/schemas/shopping/cart.update_req.json
@@ -0,0 +1,36 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/cart.update_req.json",
+  "name": "dev.ucp.shopping.cart",
+  "version": "2026-01-11",
+  "title": "Cart Update Request",
+  "description": "Cart schema for multi-item baskets prior to checkout.",
+  "type": "object",
+  "required": [
+    "id",
+    "line_items",
+    "currency"
+  ],
+  "additionalProperties": true,
+  "properties": {
+    "id": {
+      "type": "string",
+      "description": "Unique cart identifier."
+    },
+    "line_items": {
+      "type": "array",
+      "items": {
+        "$ref": "types/line_item.update_req.json"
+      },
+      "description": "List of line items in the cart."
+    },
+    "buyer": {
+      "$ref": "types/buyer.json",
+      "description": "Representation of the buyer."
+    },
+    "currency": {
+      "type": "string",
+      "description": "ISO 4217 currency code."
+    }
+  }
+}
diff --git a/spec/schemas/shopping/cart_resp.json b/spec/schemas/shopping/cart_resp.json
new file mode 100644
index 000000000..de2f7f7ea
--- /dev/null
+++ b/spec/schemas/shopping/cart_resp.json
@@ -0,0 +1,83 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/cart.json",
+  "name": "dev.ucp.shopping.cart",
+  "version": "2026-01-11",
+  "title": "Cart Response",
+  "description": "Cart schema for multi-item baskets prior to checkout.",
+  "type": "object",
+  "required": [
+    "ucp",
+    "id",
+    "line_items",
+    "status",
+    "currency",
+    "totals"
+  ],
+  "additionalProperties": true,
+  "properties": {
+    "ucp": {
+      "$ref": "../ucp.json#/$defs/response_cart"
+    },
+    "id": {
+      "type": "string",
+      "description": "Unique cart identifier."
+    },
+    "line_items": {
+      "type": "array",
+      "items": {
+        "$ref": "types/line_item_resp.json"
+      },
+      "description": "List of line items in the cart."
+    },
+    "buyer": {
+      "$ref": "types/buyer.json",
+      "description": "Representation of the buyer."
+    },
+    "status": {
+      "type": "string",
+      "enum": [
+        "active",
+        "ready_for_checkout",
+        "converted",
+        "abandoned",
+        "expired"
+      ],
+      "description": "Cart lifecycle status."
+    },
+    "currency": {
+      "type": "string",
+      "description": "ISO 4217 currency code."
+    },
+    "totals": {
+      "type": "array",
+      "items": {
+        "$ref": "types/total_resp.json"
+      },
+      "description": "Different cart totals."
+    },
+    "messages": {
+      "type": "array",
+      "items": {
+        "$ref": "types/message.json"
+      },
+      "description": "Messages about cart state."
+    },
+    "links": {
+      "type": "array",
+      "items": {
+        "$ref": "types/link.json"
+      },
+      "description": "Links to policies and terms."
+    },
+    "expires_at": {
+      "type": "string",
+      "format": "date-time",
+      "description": "RFC 3339 expiry timestamp."
+    },
+    "checkout_id": {
+      "type": "string",
+      "description": "Checkout session created from this cart, if converted."
+    }
+  }
+}
diff --git a/spec/schemas/shopping/loyalty.create_req.json b/spec/schemas/shopping/loyalty.create_req.json
new file mode 100644
index 000000000..2e68f1fe0
--- /dev/null
+++ b/spec/schemas/shopping/loyalty.create_req.json
@@ -0,0 +1,82 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/loyalty.create_req.json",
+  "name": "dev.ucp.shopping.loyalty",
+  "version": "2026-01-11",
+  "title": "Loyalty Extension Create Request",
+  "description": "Extends Checkout with loyalty program linkage and benefit application.",
+  "$defs": {
+    "benefit": {
+      "type": "object",
+      "description": "A loyalty benefit applied to the checkout.",
+      "required": [
+        "type",
+        "amount"
+      ],
+      "properties": {
+        "type": {
+          "type": "string",
+          "description": "Benefit type (e.g., points_redemption, member_discount, free_shipping)."
+        },
+        "amount": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Benefit value in minor (cents) currency units or points, depending on type."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable description of the applied benefit."
+        }
+      }
+    },
+    "redemption": {
+      "type": "object",
+      "description": "Requested redemption inputs from the platform.",
+      "properties": {
+        "points": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Number of points to redeem."
+        }
+      }
+    },
+    "loyalty_object": {
+      "type": "object",
+      "description": "Loyalty program linkage and benefit application state.",
+      "required": [
+        "program_id"
+      ],
+      "properties": {
+        "program_id": {
+          "type": "string",
+          "description": "Identifier for the loyalty program."
+        },
+        "member_id": {
+          "type": "string",
+          "description": "Member identifier for the linked account."
+        },
+        "redemption": {
+          "$ref": "#/$defs/redemption",
+          "description": "Requested redemptions submitted by the platform."
+        }
+      }
+    },
+    "checkout": {
+      "title": "Checkout with Loyalty Create Request",
+      "description": "Checkout extended with loyalty capability.",
+      "allOf": [
+        {
+          "$ref": "checkout.create_req.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "loyalty": {
+              "$ref": "#/$defs/loyalty_object"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/shopping/loyalty.update_req.json b/spec/schemas/shopping/loyalty.update_req.json
new file mode 100644
index 000000000..9df5db36f
--- /dev/null
+++ b/spec/schemas/shopping/loyalty.update_req.json
@@ -0,0 +1,82 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/loyalty.update_req.json",
+  "name": "dev.ucp.shopping.loyalty",
+  "version": "2026-01-11",
+  "title": "Loyalty Extension Update Request",
+  "description": "Extends Checkout with loyalty program linkage and benefit application.",
+  "$defs": {
+    "benefit": {
+      "type": "object",
+      "description": "A loyalty benefit applied to the checkout.",
+      "required": [
+        "type",
+        "amount"
+      ],
+      "properties": {
+        "type": {
+          "type": "string",
+          "description": "Benefit type (e.g., points_redemption, member_discount, free_shipping)."
+        },
+        "amount": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Benefit value in minor (cents) currency units or points, depending on type."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable description of the applied benefit."
+        }
+      }
+    },
+    "redemption": {
+      "type": "object",
+      "description": "Requested redemption inputs from the platform.",
+      "properties": {
+        "points": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Number of points to redeem."
+        }
+      }
+    },
+    "loyalty_object": {
+      "type": "object",
+      "description": "Loyalty program linkage and benefit application state.",
+      "required": [
+        "program_id"
+      ],
+      "properties": {
+        "program_id": {
+          "type": "string",
+          "description": "Identifier for the loyalty program."
+        },
+        "member_id": {
+          "type": "string",
+          "description": "Member identifier for the linked account."
+        },
+        "redemption": {
+          "$ref": "#/$defs/redemption",
+          "description": "Requested redemptions submitted by the platform."
+        }
+      }
+    },
+    "checkout": {
+      "title": "Checkout with Loyalty Update Request",
+      "description": "Checkout extended with loyalty capability.",
+      "allOf": [
+        {
+          "$ref": "checkout.update_req.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "loyalty": {
+              "$ref": "#/$defs/loyalty_object"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/shopping/loyalty_resp.json b/spec/schemas/shopping/loyalty_resp.json
new file mode 100644
index 000000000..d8b06ab87
--- /dev/null
+++ b/spec/schemas/shopping/loyalty_resp.json
@@ -0,0 +1,108 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/loyalty.json",
+  "name": "dev.ucp.shopping.loyalty",
+  "version": "2026-01-11",
+  "title": "Loyalty Extension Response",
+  "description": "Extends Checkout with loyalty program linkage and benefit application.",
+  "$defs": {
+    "benefit": {
+      "type": "object",
+      "description": "A loyalty benefit applied to the checkout.",
+      "required": [
+        "type",
+        "amount"
+      ],
+      "properties": {
+        "type": {
+          "type": "string",
+          "description": "Benefit type (e.g., points_redemption, member_discount, free_shipping)."
+        },
+        "amount": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Benefit value in minor (cents) currency units or points, depending on type."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable description of the applied benefit."
+        }
+      }
+    },
+    "redemption": {
+      "type": "object",
+      "description": "Requested redemption inputs from the platform.",
+      "properties": {
+        "points": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Number of points to redeem."
+        }
+      }
+    },
+    "loyalty_object": {
+      "type": "object",
+      "description": "Loyalty program linkage and benefit application state.",
+      "required": [
+        "program_id"
+      ],
+      "properties": {
+        "program_id": {
+          "type": "string",
+          "description": "Identifier for the loyalty program."
+        },
+        "member_id": {
+          "type": "string",
+          "description": "Member identifier for the linked account."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "linked",
+            "unlinked",
+            "pending",
+            "ineligible"
+          ],
+          "description": "Loyalty linkage status for this checkout."
+        },
+        "tier": {
+          "type": "string",
+          "description": "Member tier or segment label."
+        },
+        "points_balance": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Member points balance at the time of checkout."
+        },
+        "benefits_applied": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/benefit"
+          },
+          "description": "Benefits applied to the checkout by the business."
+        },
+        "redemption": {
+          "$ref": "#/$defs/redemption",
+          "description": "Requested redemptions submitted by the platform."
+        }
+      }
+    },
+    "checkout": {
+      "title": "Checkout with Loyalty Response",
+      "description": "Checkout extended with loyalty capability.",
+      "allOf": [
+        {
+          "$ref": "checkout_resp.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "loyalty": {
+              "$ref": "#/$defs/loyalty_object"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/shopping/post_order.create_req.json b/spec/schemas/shopping/post_order.create_req.json
new file mode 100644
index 000000000..081096e74
--- /dev/null
+++ b/spec/schemas/shopping/post_order.create_req.json
@@ -0,0 +1,218 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/post_order.create_req.json",
+  "name": "dev.ucp.shopping.post_order",
+  "version": "2026-01-11",
+  "title": "Post-Order Extension Create Request",
+  "description": "Extends Order with standardized return, exchange, and refund events.",
+  "$defs": {
+    "line_item_reference": {
+      "type": "object",
+      "required": [
+        "id",
+        "quantity"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Line item ID reference."
+        },
+        "quantity": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Quantity affected by this event."
+        }
+      }
+    },
+    "return_event": {
+      "type": "object",
+      "description": "Return event for items after purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Return event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Return status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the return."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable return reason (e.g., defective, wrong_size)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable return reason."
+        }
+      }
+    },
+    "exchange_event": {
+      "type": "object",
+      "description": "Exchange event for swapping items post-purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Exchange event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Exchange status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities being exchanged."
+        },
+        "replacement_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Replacement line items and quantities."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable exchange reason (e.g., wrong_size, color_change)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable exchange reason."
+        }
+      }
+    },
+    "refund_event": {
+      "type": "object",
+      "description": "Refund event for returned or canceled items.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at",
+        "amount"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Refund event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "pending",
+            "completed",
+            "failed"
+          ],
+          "description": "Refund status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this refund occurred."
+        },
+        "amount": {
+          "type": "integer",
+          "description": "Refund amount in minor (cents) currency units."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the refund."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable refund reason (e.g., return_completed)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable refund reason."
+        }
+      }
+    },
+    "post_order_object": {
+      "type": "object",
+      "description": "Structured post-order events for returns, exchanges, and refunds.",
+      "properties": {
+        "returns": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/return_event"
+          },
+          "description": "Return events for the order."
+        },
+        "exchanges": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/exchange_event"
+          },
+          "description": "Exchange events for the order."
+        },
+        "refunds": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/refund_event"
+          },
+          "description": "Refund events for the order."
+        }
+      }
+    },
+    "order": {
+      "title": "Order with Post-Order Events Create Request",
+      "description": "Order extended with post-order management data.",
+      "allOf": [
+        {
+          "$ref": "order.json"
+        },
+        {
+          "type": "object",
+          "properties": {}
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/shopping/post_order.update_req.json b/spec/schemas/shopping/post_order.update_req.json
new file mode 100644
index 000000000..81799dcb7
--- /dev/null
+++ b/spec/schemas/shopping/post_order.update_req.json
@@ -0,0 +1,218 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/post_order.update_req.json",
+  "name": "dev.ucp.shopping.post_order",
+  "version": "2026-01-11",
+  "title": "Post-Order Extension Update Request",
+  "description": "Extends Order with standardized return, exchange, and refund events.",
+  "$defs": {
+    "line_item_reference": {
+      "type": "object",
+      "required": [
+        "id",
+        "quantity"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Line item ID reference."
+        },
+        "quantity": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Quantity affected by this event."
+        }
+      }
+    },
+    "return_event": {
+      "type": "object",
+      "description": "Return event for items after purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Return event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Return status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the return."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable return reason (e.g., defective, wrong_size)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable return reason."
+        }
+      }
+    },
+    "exchange_event": {
+      "type": "object",
+      "description": "Exchange event for swapping items post-purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Exchange event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Exchange status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities being exchanged."
+        },
+        "replacement_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Replacement line items and quantities."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable exchange reason (e.g., wrong_size, color_change)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable exchange reason."
+        }
+      }
+    },
+    "refund_event": {
+      "type": "object",
+      "description": "Refund event for returned or canceled items.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at",
+        "amount"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Refund event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "pending",
+            "completed",
+            "failed"
+          ],
+          "description": "Refund status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this refund occurred."
+        },
+        "amount": {
+          "type": "integer",
+          "description": "Refund amount in minor (cents) currency units."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the refund."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable refund reason (e.g., return_completed)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable refund reason."
+        }
+      }
+    },
+    "post_order_object": {
+      "type": "object",
+      "description": "Structured post-order events for returns, exchanges, and refunds.",
+      "properties": {
+        "returns": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/return_event"
+          },
+          "description": "Return events for the order."
+        },
+        "exchanges": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/exchange_event"
+          },
+          "description": "Exchange events for the order."
+        },
+        "refunds": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/refund_event"
+          },
+          "description": "Refund events for the order."
+        }
+      }
+    },
+    "order": {
+      "title": "Order with Post-Order Events Update Request",
+      "description": "Order extended with post-order management data.",
+      "allOf": [
+        {
+          "$ref": "order.json"
+        },
+        {
+          "type": "object",
+          "properties": {}
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/shopping/post_order_resp.json b/spec/schemas/shopping/post_order_resp.json
new file mode 100644
index 000000000..11117201d
--- /dev/null
+++ b/spec/schemas/shopping/post_order_resp.json
@@ -0,0 +1,222 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/post_order.json",
+  "name": "dev.ucp.shopping.post_order",
+  "version": "2026-01-11",
+  "title": "Post-Order Extension Response",
+  "description": "Extends Order with standardized return, exchange, and refund events.",
+  "$defs": {
+    "line_item_reference": {
+      "type": "object",
+      "required": [
+        "id",
+        "quantity"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Line item ID reference."
+        },
+        "quantity": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Quantity affected by this event."
+        }
+      }
+    },
+    "return_event": {
+      "type": "object",
+      "description": "Return event for items after purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Return event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Return status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the return."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable return reason (e.g., defective, wrong_size)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable return reason."
+        }
+      }
+    },
+    "exchange_event": {
+      "type": "object",
+      "description": "Exchange event for swapping items post-purchase.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Exchange event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "requested",
+            "approved",
+            "rejected",
+            "in_progress",
+            "completed",
+            "canceled"
+          ],
+          "description": "Exchange status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this event occurred."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities being exchanged."
+        },
+        "replacement_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Replacement line items and quantities."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable exchange reason (e.g., wrong_size, color_change)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable exchange reason."
+        }
+      }
+    },
+    "refund_event": {
+      "type": "object",
+      "description": "Refund event for returned or canceled items.",
+      "required": [
+        "id",
+        "status",
+        "occurred_at",
+        "amount"
+      ],
+      "properties": {
+        "id": {
+          "type": "string",
+          "description": "Refund event identifier."
+        },
+        "status": {
+          "type": "string",
+          "enum": [
+            "pending",
+            "completed",
+            "failed"
+          ],
+          "description": "Refund status."
+        },
+        "occurred_at": {
+          "type": "string",
+          "format": "date-time",
+          "description": "RFC 3339 timestamp when this refund occurred."
+        },
+        "amount": {
+          "type": "integer",
+          "description": "Refund amount in minor (cents) currency units."
+        },
+        "line_items": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/line_item_reference"
+          },
+          "description": "Line items and quantities included in the refund."
+        },
+        "reason_code": {
+          "type": "string",
+          "description": "Machine-readable refund reason (e.g., return_completed)."
+        },
+        "description": {
+          "type": "string",
+          "description": "Human-readable refund reason."
+        }
+      }
+    },
+    "post_order_object": {
+      "type": "object",
+      "description": "Structured post-order events for returns, exchanges, and refunds.",
+      "properties": {
+        "returns": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/return_event"
+          },
+          "description": "Return events for the order."
+        },
+        "exchanges": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/exchange_event"
+          },
+          "description": "Exchange events for the order."
+        },
+        "refunds": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/refund_event"
+          },
+          "description": "Refund events for the order."
+        }
+      }
+    },
+    "order": {
+      "title": "Order with Post-Order Events Response",
+      "description": "Order extended with post-order management data.",
+      "allOf": [
+        {
+          "$ref": "order.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "post_order": {
+              "$ref": "#/$defs/post_order_object"
+            }
+          }
+        }
+      ]
+    }
+  }
+}
diff --git a/spec/schemas/ucp.json b/spec/schemas/ucp.json
index 0067c8a85..0e4134dc9 100644
--- a/spec/schemas/ucp.json
+++ b/spec/schemas/ucp.json
@@ -62,6 +62,27 @@
         }
       }
     },
+    "response_cart": {
+      "title": "UCP Cart Response",
+      "description": "UCP metadata for cart responses.",
+      "type": "object",
+      "required": [
+        "version",
+        "capabilities"
+      ],
+      "properties": {
+        "version": {
+          "$ref": "#/$defs/version"
+        },
+        "capabilities": {
+          "type": "array",
+          "description": "Active capabilities for this response.",
+          "items": {
+            "$ref": "capability.json#/$defs/response"
+          }
+        }
+      }
+    },
     "response_order": {
       "title": "UCP Order Response",
       "description": "UCP metadata for order responses. No payment handlers needed post-purchase.",
diff --git a/spec/services/shopping/rest.openapi.json b/spec/services/shopping/rest.openapi.json
index 83b92f6d6..4c5728156 100644
--- a/spec/services/shopping/rest.openapi.json
+++ b/spec/services/shopping/rest.openapi.json
@@ -18,6 +18,194 @@
     }
   ],
   "paths": {
+    "/carts": {
+      "post": {
+        "operationId": "create_cart",
+        "summary": "Create Cart",
+        "description": "Create a cart session. Called when a user begins building a basket.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/idempotency_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/cart_create_request"
+              }
+            }
+          }
+        },
+        "responses": {
+          "201": {
+            "description": "Cart created",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart_response"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/carts/{id}": {
+      "parameters": [
+        {
+          "$ref": "#/components/parameters/cart_id_path"
+        }
+      ],
+      "get": {
+        "operationId": "get_cart",
+        "summary": "Get Cart",
+        "description": "Get the latest state of a cart session.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Cart retrieved",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart_response"
+                }
+              }
+            }
+          }
+        }
+      },
+      "put": {
+        "operationId": "update_cart",
+        "summary": "Update Cart",
+        "description": "If an optional field is provided in the request body, its value is treated as a complete replacement for the corresponding data. If optional field is omitted, then current cart session is unchanged.",
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/authorization"
+          },
+          {
+            "$ref": "#/components/parameters/x_api_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_signature"
+          },
+          {
+            "$ref": "#/components/parameters/idempotency_key"
+          },
+          {
+            "$ref": "#/components/parameters/request_id"
+          },
+          {
+            "$ref": "#/components/parameters/user_agent"
+          },
+          {
+            "$ref": "#/components/parameters/content_type"
+          },
+          {
+            "$ref": "#/components/parameters/accept"
+          },
+          {
+            "$ref": "#/components/parameters/accept_language"
+          },
+          {
+            "$ref": "#/components/parameters/accept_encoding"
+          }
+        ],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/cart_update_request"
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "Cart updated",
+            "headers": {
+              "X-Detached-JWT": {
+                "$ref": "#/components/headers/x_detached_jwt"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/cart_response"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/checkout-sessions": {
       "post": {
         "operationId": "create_checkout",
@@ -425,6 +613,15 @@
   },
   "components": {
     "parameters": {
+      "cart_id_path": {
+        "name": "id",
+        "in": "path",
+        "required": true,
+        "description": "The unique identifier of the cart.",
+        "schema": {
+          "type": "string"
+        }
+      },
       "checkout_session_id_path": {
         "name": "id",
         "in": "path",
@@ -546,6 +743,15 @@
       "ucp": {
         "$ref": "https://ucp.dev/schemas/ucp.json"
       },
+      "cart_response": {
+        "$ref": "https://ucp.dev/schemas/shopping/cart_resp.json"
+      },
+      "cart_create_request": {
+        "$ref": "https://ucp.dev/schemas/shopping/cart.create_req.json"
+      },
+      "cart_update_request": {
+        "$ref": "https://ucp.dev/schemas/shopping/cart.update_req.json"
+      },
       "checkout_response": {
         "$ref": "https://ucp.dev/schemas/shopping/checkout_resp.json"
       },