diff --git a/devcycle_python_sdk/models/user.py b/devcycle_python_sdk/models/user.py
index 57d4c56..3110ead 100644
--- a/devcycle_python_sdk/models/user.py
+++ b/devcycle_python_sdk/models/user.py
@@ -101,22 +101,33 @@ def create_user_from_context(
     ) -> "DevCycleUser":
         """
         Builds a DevCycleUser instance from the evaluation context. Will raise a TargetingKeyMissingError if
-        the context does not contain a valid targeting key or user_id attribute
+        the context does not contain a valid targeting key, user_id, or userId attribute
 
         :param context: The evaluation context to build the user from
         :return: A DevCycleUser instance
         """
         user_id = None
+        user_id_source = None
 
         if context:
             if context.targeting_key:
                 user_id = context.targeting_key
+                user_id_source = "targeting_key"
             elif context.attributes and "user_id" in context.attributes.keys():
                 user_id = context.attributes["user_id"]
+                user_id_source = "user_id"
+            elif context.attributes and "userId" in context.attributes.keys():
+                user_id = context.attributes["userId"]
+                user_id_source = "userId"
 
-        if not user_id or not isinstance(user_id, str):
+        if not user_id:
             raise TargetingKeyMissingError(
-                "DevCycle: Evaluation context does not contain a valid targeting key or user_id attribute"
+                "DevCycle: Evaluation context does not contain a valid targeting key, user_id, or userId attribute"
+            )
+
+        if not isinstance(user_id, str):
+            raise TargetingKeyMissingError(
+                f"DevCycle: {user_id_source} must be a string, got {type(user_id).__name__}"
             )
 
         user = DevCycleUser(user_id=user_id)
@@ -124,7 +135,8 @@ def create_user_from_context(
         private_custom_data: Dict[str, Any] = {}
         if context and context.attributes:
             for key, value in context.attributes.items():
-                if key == "user_id":
+                # Skip user_id, userId, and targeting_key - these are reserved for user ID mapping
+                if key in ("user_id", "userId", "targeting_key"):
                     continue
 
                 if value is not None:
diff --git a/test/models/test_user.py b/test/models/test_user.py
index 0bda339..334cab8 100644
--- a/test/models/test_user.py
+++ b/test/models/test_user.py
@@ -53,6 +53,96 @@ def test_create_user_from_context_only_user_id(self):
         self.assertIsNotNone(user)
         self.assertEqual(user.user_id, test_user_id)
 
+    def test_create_user_from_context_with_user_id_attribute(self):
+        """Comprehensive test for userId attribute support including priority, functionality, and error cases"""
+        targeting_key_id = "targeting-12345"
+        user_id = "user_id-12345"
+        user_id_attr = "userId-12345"
+
+        # Test 1: userId as the only user ID source
+        context = EvaluationContext(
+            targeting_key=None, attributes={"userId": user_id_attr}
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertIsNotNone(user)
+        self.assertEqual(user.user_id, user_id_attr)
+
+        # Test 2: Priority order - targeting_key > user_id > userId
+        context = EvaluationContext(
+            targeting_key=targeting_key_id,
+            attributes={"user_id": user_id, "userId": user_id_attr},
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertEqual(user.user_id, targeting_key_id)
+
+        context = EvaluationContext(
+            targeting_key=None, attributes={"user_id": user_id, "userId": user_id_attr}
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertEqual(user.user_id, user_id)
+
+        # Test 3: Error cases - invalid userId types
+        with self.assertRaises(TargetingKeyMissingError):
+            DevCycleUser.create_user_from_context(
+                EvaluationContext(targeting_key=None, attributes={"userId": 12345})
+            )
+
+        with self.assertRaises(TargetingKeyMissingError):
+            DevCycleUser.create_user_from_context(
+                EvaluationContext(targeting_key=None, attributes={"userId": None})
+            )
+
+    def test_create_user_from_context_user_id_exclusion(self):
+        """Test that all user ID fields (targeting_key, user_id, userId) are excluded from custom data"""
+        targeting_key_id = "targeting-12345"
+        user_id = "user_id-12345"
+        user_id_attr = "userId-12345"
+
+        # Test exclusion when targeting_key is used
+        context = EvaluationContext(
+            targeting_key=targeting_key_id,
+            attributes={
+                "user_id": user_id,
+                "userId": user_id_attr,
+                "targeting_key": "should-be-excluded",
+                "other_field": "value",
+            },
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertEqual(user.user_id, targeting_key_id)
+        self.assertIsNotNone(user.customData)
+        self.assertNotIn("user_id", user.customData)
+        self.assertNotIn("userId", user.customData)
+        self.assertNotIn("targeting_key", user.customData)
+        self.assertEqual(user.customData["other_field"], "value")
+
+        # Test exclusion when user_id is used
+        context = EvaluationContext(
+            targeting_key=None,
+            attributes={
+                "user_id": user_id,
+                "userId": user_id_attr,
+                "other_field": "value",
+            },
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertEqual(user.user_id, user_id)
+        self.assertIsNotNone(user.customData)
+        self.assertNotIn("user_id", user.customData)
+        self.assertNotIn("userId", user.customData)
+        self.assertEqual(user.customData["other_field"], "value")
+
+        # Test exclusion when userId is used
+        context = EvaluationContext(
+            targeting_key=None,
+            attributes={"userId": user_id_attr, "other_field": "value"},
+        )
+        user = DevCycleUser.create_user_from_context(context)
+        self.assertEqual(user.user_id, user_id_attr)
+        self.assertIsNotNone(user.customData)
+        self.assertNotIn("userId", user.customData)
+        self.assertEqual(user.customData["other_field"], "value")
+
     def test_create_user_from_context_with_attributes(self):
         test_user_id = "12345"
         context = EvaluationContext(