diff --git a/rest/python/client/flower_shop/sample_output/happy_path_dialog.md b/rest/python/client/flower_shop/sample_output/happy_path_dialog.md
index 4b482ca..a9b4b35 100644
--- a/rest/python/client/flower_shop/sample_output/happy_path_dialog.md
+++ b/rest/python/client/flower_shop/sample_output/happy_path_dialog.md
@@ -1353,9 +1353,9 @@ export RESPONSE=$(curl -s -X POST $SERVER_URL/checkout-sessions/$CHECKOUT_ID/com
   -H 'request-id: f6478591-3679-4459-baa7-7ecddb6a5c95' \
   -H 'Content-Type: application/json' \
   -d '{
-  "risk_signals": {
-    "ip": "127.0.0.1",
-    "browser": "python-httpx"
+  "signals": {
+    "com.example.ip": "127.0.0.1",
+    "com.example.browser": "python-httpx"
   },
   "payment": {
     "instruments": [
diff --git a/rest/python/client/flower_shop/simple_happy_path_client.py b/rest/python/client/flower_shop/simple_happy_path_client.py
index 3527b3b..2913b8e 100644
--- a/rest/python/client/flower_shop/simple_happy_path_client.py
+++ b/rest/python/client/flower_shop/simple_happy_path_client.py
@@ -861,9 +861,9 @@ def main() -> int:
           }
         ]
       },
-      "risk_signals": {
-        "ip": "127.0.0.1",
-        "browser": "python-httpx",
+      "signals": {
+        "com.example.ip": "127.0.0.1",
+        "com.example.browser": "python-httpx",
       },
     }
 
diff --git a/rest/python/server/integration_test.py b/rest/python/server/integration_test.py
index 6f9d095..465d991 100644
--- a/rest/python/server/integration_test.py
+++ b/rest/python/server/integration_test.py
@@ -319,7 +319,6 @@ def _create_payment_payload(self) -> dict:
           }
         ]
       ),
-      risk_signals={},
     )
     return payload.model_dump(mode="json", exclude_none=True)
 
@@ -2224,6 +2223,132 @@ def test_checkout_readiness_lifecycle(self) -> None:
       self.assertEqual(complete_res.status_code, 200, complete_res.text)
       self.assertEqual(complete_res.json()["status"], "completed")
 
+  def _create_and_get_id(self, checkout_id: str, key: str) -> str:
+    """Create a checkout and return the server-assigned id."""
+    payload = self._create_checkout_payload(
+      checkout_id, [("rose", "Red Rose", 1000, 1)]
+    )
+    response = self.client.post(
+      "/checkout-sessions",
+      headers=self._get_headers(idempotency_key=key, request_id=key),
+      json=payload.model_dump(mode="json", exclude_none=True),
+    )
+    self.assertEqual(response.status_code, 201, response.text)
+    return self.get_resource_id(TestCheckout.model_validate(response.json()).id)
+
+  def test_complete_accepts_the_spec_shaped_body(self) -> None:
+    """Complete Checkout takes the body the spec defines.
+
+    checkout.json marks `payment` complete: required and `signals` optional.
+    There is no `risk_signals` member anywhere in the specification; it was
+    replaced by `signals` before the 2026-04-08 release. The handler declared
+    `risk_signals` as a required body member, so a spec-shaped request was
+    rejected with 422.
+    """
+    with self.client:
+      checkout_sid = self._create_and_get_id("spec_body_1", "sb1")
+      body = {
+        "payment": {
+          "instruments": [
+            {
+              "id": "instr_1",
+              "handler_id": "mock_payment_handler",
+              "type": "card",
+              "display": {"brand": "Visa", "last_digits": "1234"},
+              "credential": {"type": "token", "token": "success_token"},
+            }
+          ]
+        },
+        "signals": {"com.example.session": "xts-marker-2030"},
+      }
+      response = self.client.post(
+        f"/checkout-sessions/{checkout_sid}/complete",
+        headers=self._get_headers(idempotency_key="sb2", request_id="sb2"),
+        json=body,
+      )
+      self.assertEqual(response.status_code, 200, response.text)
+      self.assertEqual(
+        TestCheckout.model_validate(response.json()).status, "completed"
+      )
+
+  def test_complete_rejects_a_malformed_payment_with_4xx(self) -> None:
+    """A malformed payment is a client error, not a server fault.
+
+    The handler built PaymentCreateRequest inside its own body, so pydantic
+    raised a ValidationError that was not a RequestValidationError and the
+    request surfaced as a bare 500.
+    """
+    with self.client:
+      checkout_sid = self._create_and_get_id("bad_payment_1", "bp1")
+      response = self.client.post(
+        f"/checkout-sessions/{checkout_sid}/complete",
+        headers=self._get_headers(idempotency_key="bp2", request_id="bp2"),
+        json={
+          # risk_signals is sent so the request reaches the payment parsing
+          # rather than being turned away for the missing member. It is a
+          # tolerated extra once the body binds to the SDK model.
+          "risk_signals": {},
+          "payment": {
+            "instruments": [
+              {"id": "instr_1", "handler_id": 12345, "type": "card"}
+            ]
+          },
+        },
+      )
+      self.assertEqual(response.status_code, 422, response.text)
+      self.assertEqual(response.json()["ucp"]["status"], "error")
+
+  def test_complete_rejects_a_non_list_instruments_with_4xx(self) -> None:
+    """The same holds when instruments is not an array at all."""
+    with self.client:
+      checkout_sid = self._create_and_get_id("bad_payment_2", "bp3")
+      response = self.client.post(
+        f"/checkout-sessions/{checkout_sid}/complete",
+        headers=self._get_headers(idempotency_key="bp4", request_id="bp4"),
+        json={
+          "risk_signals": {},
+          "payment": {"instruments": "xts-not-a-list"},
+        },
+      )
+      self.assertEqual(response.status_code, 422, response.text)
+
+  def test_complete_carries_signals_into_the_request_identity(self) -> None:
+    """`signals` is part of the request, so it must reach the service.
+
+    The handler bound only `payment` and `risk_signals`, so a top-level
+    `signals` member was silently discarded. Reusing one idempotency key with
+    a different `signals` value is then indistinguishable from a replay. With
+    the body bound to the SDK model it is a conflict.
+    """
+    with self.client:
+      checkout_sid = self._create_and_get_id("signals_1", "sg1")
+      base = {
+        "payment": {
+          "instruments": [
+            {
+              "id": "instr_1",
+              "handler_id": "mock_payment_handler",
+              "type": "card",
+              "display": {"brand": "Visa", "last_digits": "1234"},
+              "credential": {"type": "token", "token": "success_token"},
+            }
+          ]
+        },
+      }
+      first = self.client.post(
+        f"/checkout-sessions/{checkout_sid}/complete",
+        headers=self._get_headers(idempotency_key="sg2", request_id="sg2"),
+        json=dict(base, signals={"com.example.session": "first-xts"}),
+      )
+      self.assertEqual(first.status_code, 200, first.text)
+
+      second = self.client.post(
+        f"/checkout-sessions/{checkout_sid}/complete",
+        headers=self._get_headers(idempotency_key="sg2", request_id="sg3"),
+        json=dict(base, signals={"com.example.session": "second-xts"}),
+      )
+      self.assertEqual(second.status_code, 409, second.text)
+
 
 if __name__ == "__main__":
   absltest.main()
diff --git a/rest/python/server/routes/ucp_implementation.py b/rest/python/server/routes/ucp_implementation.py
index 1a0460e..77ae161 100644
--- a/rest/python/server/routes/ucp_implementation.py
+++ b/rest/python/server/routes/ucp_implementation.py
@@ -207,8 +207,7 @@ async def update_checkout(
 
 async def complete_checkout(
   checkout_id: Annotated[str, Path(..., alias="id")],
-  payment: Annotated[dict[str, Any], Body(...)],
-  risk_signals: Annotated[dict[str, Any], Body(...)],
+  checkout_complete: Annotated[CheckoutCompleteRequest, Body(...)],
   common_headers: Annotated[
     dependencies.CommonHeaders, Depends(dependencies.common_headers)
   ],
@@ -216,18 +215,23 @@ async def complete_checkout(
   checkout_service: Annotated[
     CheckoutService, Depends(dependencies.get_checkout_service)
   ],
-  checkout_complete: Annotated[CheckoutCompleteRequest | None, Body()] = None,
 ) -> models.UnifiedCheckout:
-  """Complete Checkout Implementation."""
+  """Complete Checkout Implementation.
+
+  The body is the checkout complete projection: `payment` is required and
+  `signals` and `attribution` are optional. Binding it to the SDK model means
+  a malformed payment is answered by the framework as a 422 in the UCP
+  envelope, rather than raising inside the handler as a 500.
+  """
   del common_headers  # Unused
 
-  # Parse payment into PaymentCreateRequest
-  payment_req = PaymentCreateRequest(**payment)
+  payment_req = PaymentCreateRequest(
+    **checkout_complete.payment.model_dump(mode="json", exclude_none=True)
+  )
 
   return await checkout_service.complete_checkout(
     checkout_id,
     payment_req,
-    risk_signals,
     idempotency_key,
     checkout_complete=checkout_complete,
   )
diff --git a/rest/python/server/services/checkout_service.py b/rest/python/server/services/checkout_service.py
index f6c1669..4cc90fe 100644
--- a/rest/python/server/services/checkout_service.py
+++ b/rest/python/server/services/checkout_service.py
@@ -716,7 +716,6 @@ async def complete_checkout(
     self,
     checkout_id: str,
     payment: PaymentCreateRequest,
-    risk_signals: dict[str, Any],
     idempotency_key: str,
     checkout_complete: CheckoutCompleteRequest | None = None,
   ) -> Checkout:
@@ -724,10 +723,10 @@ async def complete_checkout(
     logger.info("Completing checkout session %s", checkout_id)
 
     # Idempotency Check
-    # Include risk_signals and checkout_complete in the hash
+    # The whole request body is part of the request identity, so signals and
+    # attribution are included through checkout_complete.
     combined_data = {
       "payment": payment.model_dump(mode="json"),
-      "risk_signals": risk_signals,
       "checkout_complete": checkout_complete.model_dump(mode="json")
       if checkout_complete
       else None,