feat: add discount at checkout feature

a2a/README.md

@@ -148,11 +148,12 @@ The Chat Client UCP Profile can be found at http://localhost:3000/profile/agent-
 2. Type `"show me cookies available in stock"` and press enter
 3. The agent will return products available in stock
 4. Click **"Add to Checkout"** for any product
-5. The agent will ask for required information such as email address, shipping address, etc.
-6. Once the required information is provided, click **"Complete Payment"**
-7. The UI shows available mock payment options
-8. Select a payment method and click **"Confirm Purchase"**
-9. The agent will create an order and return the order confirmation
+5. [Optional] Type `"apply discount code 10OFF"` to get a 10% discount
+6. The agent will ask for required information such as email address, shipping address, etc.
+7. Once the required information is provided, click **"Complete Payment"**
+8. The UI shows available mock payment options
+9. Select a payment method and click **"Confirm Purchase"**
+10. The agent will create an order and return the order confirmation
 
 </td>
 </tr>

a2a/business_agent/src/business_agent/agent.py

@@ -209,6 +209,32 @@ def get_checkout(tool_context: ToolContext) -> dict:
     }
 
 
+def apply_discount_code(tool_context: ToolContext, discount_code: str) -> dict:
+    """Apply a discount code to the checkout.
+
+    Args:
+        tool_context: The tool context for the current request.
+        discount_code: The discount code to apply.
+
+    Returns:
+        dict: Returns the response from the tool with success or error status.
+
+    """
+    checkout_id = _get_current_checkout_id(tool_context)
+
+    if not checkout_id:
+        return _create_error_response("A Checkout has not yet been created.")
+
+    if store.apply_discount(checkout_id, discount_code):
+        checkout = store.get_checkout(checkout_id)
+        return {
+            UCP_CHECKOUT_KEY: checkout.model_dump(mode="json"),
+            "status": "success",
+        }
+    else:
+        return _create_error_response("Invalid discount code.")
+
+
 def update_customer_details(
     tool_context: ToolContext,
     first_name: str,
@@ -450,7 +476,8 @@ def modify_output_after_agent(
         " search for the products and then add the matching products to"
         " checkout session.If the user asks to replace products,"
         " use remove_from_checkout and add_to_checkout tools to replace the"
-        " products to match the user request"
+        " products to match the user request. If the user provides a discount"
+        " code, apply it to the checkout."
     ),
     tools=[
         search_shopping_catalog,
@@ -461,6 +488,7 @@ def modify_output_after_agent(
         start_payment,
         update_customer_details,
         complete_checkout,
+        apply_discount_code,
     ],
     after_tool_callback=after_tool_modifier,
     after_agent_callback=modify_output_after_agent,

a2a/business_agent/src/business_agent/data/agent_card.json

@@ -77,6 +77,18 @@
       "tags": [
         "checkout"
       ]
+    },
+    {
+      "description": "Applies a discount to the current checkout session",
+      "examples": [
+        "Apply discount code 10OFF"
+      ],
+      "id": "apply_discount",
+      "name": "Apply Discount",
+      "tags": [
+        "checkout",
+        "discount"
+      ]
     }
   ],
   "url": "http://localhost:10999",

a2a/business_agent/src/business_agent/data/ucp.json

@@ -23,6 +23,13 @@
         "spec": "https://ucp.dev/specs/shopping/fulfillment",
         "schema": "https://ucp.dev/schemas/shopping/fulfillment.json",
         "extends": "dev.ucp.shopping.checkout"
+      },
+      {
+        "name": "dev.ucp.shopping.discount",
+        "version": "2026-01-11",
+        "spec": "https://ucp.dev/specs/shopping/discount",
+        "schema": "https://ucp.dev/schemas/shopping/discount.json",
+        "extends": "dev.ucp.shopping.discount"
       }
     ]
   },

a2a/business_agent/src/business_agent/store.py

@@ -76,6 +76,7 @@ def __init__(self):
         self._products = {}
         self._checkouts = {}
         self._orders = {}
+        self._discounts = {}
         self._initialize_ucp_metadata()
         self._initialize_products()
 
@@ -349,6 +350,9 @@ def _recalculate_checkout(self, checkout: Checkout) -> None:
 
         subtotal = items_base_amount - items_discount
         discount = 0
+        if checkout.id in self._discounts:
+            discount_percentage = self._discounts[checkout.id]
+            discount = int(subtotal * discount_percentage / 100)
 
         totals = [
             Total(
@@ -484,6 +488,9 @@ def start_payment(self, checkout_id: str) -> Checkout | str:
         if (
             isinstance(checkout, FulfillmentCheckout)
             and checkout.fulfillment is None
+        ) and not any(
+            line_item.item.id == "donation"
+            for line_item in checkout.line_items
         ):
             messages.append("Provide a fulfillment address")
 
@@ -557,3 +564,22 @@ def _get_fulfillment_options(self) -> list[FulfillmentOptionResponse]:
                 ],
             ),
         ]
+
+    def apply_discount(self, checkout_id: str, discount_code: str) -> bool:
+        """Apply a discount to the checkout.
+
+        Args:
+            checkout_id (str): ID of the checkout to apply the discount to.
+            discount_code (str): The discount code to apply.
+
+        Returns:
+            bool: True if the discount was applied, False otherwise.
+
+        """
+        if discount_code == "10OFF":
+            self._discounts[checkout_id] = 10
+            checkout = self.get_checkout(checkout_id)
+            if checkout:
+                self._recalculate_checkout(checkout)
+            return True
+        return False