diff --git a/samples/python/src/roles/shopping_agent/subagents/shopper/tools.py b/samples/python/src/roles/shopping_agent/subagents/shopper/tools.py
index cee16eff..aff52646 100644
--- a/samples/python/src/roles/shopping_agent/subagents/shopper/tools.py
+++ b/samples/python/src/roles/shopping_agent/subagents/shopper/tools.py
@@ -65,7 +65,8 @@ def create_intent_mandate(
           datetime.now(timezone.utc) + timedelta(days=1)
       ).isoformat(),
   )
-  tool_context.state["intent_mandate"] = intent_mandate
+  # Store as dict to ensure JSON serialization compatibility with ADK session storage
+  tool_context.state["intent_mandate"] = intent_mandate.model_dump()
   return intent_mandate
 
 
@@ -93,7 +94,7 @@ async def find_products(
   message = (
       A2aMessageBuilder()
       .add_text("Find products that match the user's IntentMandate.")
-      .add_data(INTENT_MANDATE_DATA_KEY, intent_mandate.model_dump())
+      .add_data(INTENT_MANDATE_DATA_KEY, intent_mandate)
       .add_data("risk_data", risk_data)
       .add_data("debug_mode", debug_mode)
       .add_data("shopping_agent_id", "trusted_shopping_agent")
@@ -106,7 +107,8 @@ async def find_products(
 
   tool_context.state["shopping_context_id"] = task.context_id
   cart_mandates = _parse_cart_mandates(task.artifacts)
-  tool_context.state["cart_mandates"] = cart_mandates
+  # Store as list of dicts for JSON serialization compatibility
+  tool_context.state["cart_mandates"] = [cm.model_dump() for cm in cart_mandates]
   return cart_mandates
 
 
@@ -117,7 +119,11 @@ def update_chosen_cart_mandate(cart_id: str, tool_context: ToolContext) -> str:
     cart_id: The ID of the chosen cart.
     tool_context: The ADK supplied tool context.
   """
-  cart_mandates: list[CartMandate] = tool_context.state.get("cart_mandates", [])
+  cart_mandates_data = tool_context.state.get("cart_mandates", [])
+  # Reconstruct CartMandate objects from stored dicts
+  cart_mandates: list[CartMandate] = []
+  for cm in cart_mandates_data:
+    cart_mandates.append(CartMandate.model_validate(cm))
   for cart in cart_mandates:
     print(
         f"Checking cart with ID: {cart.contents.id} with chosen ID: {cart_id}"
diff --git a/samples/python/src/roles/shopping_agent/tools.py b/samples/python/src/roles/shopping_agent/tools.py
index 1d8b7bf1..455194a3 100644
--- a/samples/python/src/roles/shopping_agent/tools.py
+++ b/samples/python/src/roles/shopping_agent/tools.py
@@ -76,7 +76,8 @@ async def update_cart(
       _parse_cart_mandates(task.artifacts)
   )
 
-  tool_context.state["cart_mandate"] = updated_cart_mandate
+  # Store as dict for JSON serialization compatibility
+  tool_context.state["cart_mandate"] = updated_cart_mandate.model_dump()
   tool_context.state["shipping_address"] = shipping_address
 
   return updated_cart_mandate
@@ -163,7 +164,8 @@ def store_receipt_if_present(task, tool_context: ToolContext) -> None:
   )
   if payment_receipts:
     payment_receipt = artifact_utils.only(payment_receipts)
-    tool_context.state["payment_receipt"] = payment_receipt
+    # Store as dict for JSON serialization compatibility
+    tool_context.state["payment_receipt"] = payment_receipt.model_dump()
 
 
 def create_payment_mandate(
@@ -181,7 +183,9 @@ def create_payment_mandate(
   Returns:
     The payment mandate.
   """
-  cart_mandate = tool_context.state["cart_mandate"]
+  cart_mandate_data = tool_context.state["cart_mandate"]
+  # Reconstruct CartMandate from stored dict
+  cart_mandate = CartMandate.model_validate(cart_mandate_data)
 
   payment_request = cart_mandate.contents.payment_request
   shipping_address = tool_context.state["shipping_address"]
@@ -215,7 +219,8 @@ def create_payment_mandate(
       ),
   )
 
-  tool_context.state["payment_mandate"] = payment_mandate
+  # Store as dict for JSON serialization compatibility
+  tool_context.state["payment_mandate"] = payment_mandate.model_dump()
   return payment_mandate
 
 
@@ -238,8 +243,11 @@ def sign_mandates_on_user_device(tool_context: ToolContext) -> str:
   Returns:
       A string representing the simulated user authorization signature (JWT).
   """
-  payment_mandate: PaymentMandate = tool_context.state["payment_mandate"]
-  cart_mandate: CartMandate = tool_context.state["cart_mandate"]
+  payment_mandate_data = tool_context.state["payment_mandate"]
+  cart_mandate_data = tool_context.state["cart_mandate"]
+  # Reconstruct Pydantic objects from stored dicts
+  payment_mandate = PaymentMandate.model_validate(payment_mandate_data)
+  cart_mandate = CartMandate.model_validate(cart_mandate_data)
   cart_mandate_hash = _generate_cart_mandate_hash(cart_mandate)
   payment_mandate_hash = _generate_payment_mandate_hash(
       payment_mandate.payment_mandate_contents
@@ -250,7 +258,8 @@ def sign_mandates_on_user_device(tool_context: ToolContext) -> str:
   payment_mandate.user_authorization = (
       cart_mandate_hash + "_" + payment_mandate_hash
   )
-  tool_context.state["signed_payment_mandate"] = payment_mandate
+  # Store as dict for JSON serialization compatibility
+  tool_context.state["signed_payment_mandate"] = payment_mandate.model_dump()
   return payment_mandate.user_authorization