diff --git a/.cspell/custom-words.txt b/.cspell/custom-words.txt
index 0a4b07d..bbd457d 100644
--- a/.cspell/custom-words.txt
+++ b/.cspell/custom-words.txt
@@ -1,78 +1,80 @@
 # cspell-specific custom words related to UCP
+absl
+absltest
+adyen
 Adyen
+agentic
 Alam
 Amex
 Ant
 Anytown
+atok
+backorder
 Backordered
 Braintree
 Carrefour
 Centricity
+checkout
 Chewy
 Commerce
-Credentialless
-Depot
-EWALLET
-Etsy
-Flipkart
-Gap
-GitHub
-Google
-Gpay
-Kroger
-Lowe's
-Macy's
-Mastercard
-Paymentech
-Paypal
-Preorders
-Queensway
-Sephora
-Shopify
-Shopee
-Stripe
-Target
-UCP
-Ulta
-Visa
-Wayfair
-Worldpay
-Zalando
-adyen
-agentic
-atok
-backorder
-checkout
-credentialless
 credentialization
+credentialless
+Credentialless
 datamodel
+Depot
 dpan
+Etsy
 ewallet
+EWALLET
+Flipkart
 fontawesome
 fpan
 fulfillable
+Gap
+GitHub
+Google
 gpay
+Gpay
 ingestions
 inlinehilite
+Kroger
 linenums
 llmstxt
+Lowe's
+Macy's
 mastercard
+Mastercard
 mkdocs
 mtok
 openapi
 openrpc
+Paymentech
 paypal
+Paypal
 permissionless
 preorders
+Preorders
 proto
 protobuf
 pymdownx
+Queensway
 renderable
 repudiable
 schemas
 sdjwt
+Sephora
+Shopee
 shopify
+Shopify
+Stripe
 superfences
+Target
+UCP
+Ulta
+Visa
 vulnz
+Wayfair
+Worldpay
 yaml
-yml
\ No newline at end of file
+yml
+Zalando
diff --git a/ap2_test.py b/ap2_test.py
index 7deb90c..66dc36e 100644
--- a/ap2_test.py
+++ b/ap2_test.py
@@ -88,6 +88,56 @@ def test_ap2_mandate_completion(self) -> None:
       msg="Checkout status not 'completed'",
     )
 
+  def test_ap2_mandate_invalid_signature(self) -> None:
+    """Test checkout completion failure with invalid AP2 signature.
+
+    Given a ready-to-complete checkout session,
+    When a completion request is made with an invalid ap2 mandate signature,
+    Then the request should fail with status 400 and error code
+    'mandate_invalid_signature'.
+    """
+    response_json = self.create_checkout_session()
+    checkout_id = checkout.Checkout(**response_json).id
+
+    credential = token_credential_resp.TokenCredentialResponse(
+      type="token", token="success_token"
+    )
+    instr = payment_instrument.PaymentInstrument(
+      root=card_payment_instrument.CardPaymentInstrument(
+        id="instr_1",
+        brand="visa",
+        last_digits="4242",
+        handler_id="mock_payment_handler",
+        handler_name="mock_payment_handler",
+        type="card",
+        credential=credential,
+      )
+    )
+    payment_data = instr.root.model_dump(mode="json", exclude_none=True)
+
+    # Use trigger string for mock verification failure
+    mandate = CheckoutMandate(root="header.payload.invalid_signature~kb_sig")
+    ap2_data = Ap2CompleteRequest(checkout_mandate=mandate)
+
+    payment_payload = {
+      "payment_data": payment_data,
+      "risk_signals": {},
+      "ap2": ap2_data.model_dump(mode="json", exclude_none=True),
+    }
+
+    response = self.client.post(
+      f"/checkout-sessions/{checkout_id}/complete",
+      json=payment_payload,
+      headers=integration_test_utils.get_headers(),
+    )
+
+    self.assert_response_status(response, 400)
+    self.assertEqual(
+      response.json().get("code"),
+      "mandate_invalid_signature",
+      msg="Error code should be 'mandate_invalid_signature'",
+    )
+
 
 if __name__ == "__main__":
   absltest.main()
diff --git a/protocol_test.py b/protocol_test.py
index 7ce69e0..761d2bf 100644
--- a/protocol_test.py
+++ b/protocol_test.py
@@ -110,17 +110,17 @@ def test_version_negotiation(self):
     profile = UcpDiscoveryProfile(**discovery_resp.json())
     shopping_service = profile.ucp.services.root["dev.ucp.shopping"]
     self.assertIsNotNone(
-        shopping_service, "Shopping service not found in discovery"
+      shopping_service, "Shopping service not found in discovery"
     )
     self.assertIsNotNone(
-        shopping_service.rest, "REST config not found for shopping service"
+      shopping_service.rest, "REST config not found for shopping service"
     )
     self.assertIsNotNone(
-        shopping_service.rest.endpoint,
-        "Endpoint not found for shopping service",
+      shopping_service.rest.endpoint,
+      "Endpoint not found for shopping service",
     )
     checkout_sessions_url = (
-        f"{str(shopping_service.rest.endpoint).rstrip('/')}/checkout-sessions"
+      f"{str(shopping_service.rest.endpoint).rstrip('/')}/checkout-sessions"
     )
 
     create_payload = self.create_checkout_payload()