diff --git a/src/aks-preview/HISTORY.rst b/src/aks-preview/HISTORY.rst
index 79b399496a8..ba2aa58f128 100644
--- a/src/aks-preview/HISTORY.rst
+++ b/src/aks-preview/HISTORY.rst
@@ -11,6 +11,7 @@ To release a new version, please select a new version number (usually plus 1 to
 
 Pending
 +++++++
+* Fix monitoring addon key casing compatibility with azure-cli/acs
 
 19.0.0b28
 +++++++
diff --git a/src/aks-preview/azext_aks_preview/managed_cluster_decorator.py b/src/aks-preview/azext_aks_preview/managed_cluster_decorator.py
index af1d1ff1615..e91cc201ec5 100644
--- a/src/aks-preview/azext_aks_preview/managed_cluster_decorator.py
+++ b/src/aks-preview/azext_aks_preview/managed_cluster_decorator.py
@@ -7597,21 +7597,9 @@ def _setup_azure_monitor_logs(self, mc: ManagedCluster) -> None:
             mc.addon_profiles = {}
 
         CONST_MONITORING_ADDON_NAME = addon_consts.get("CONST_MONITORING_ADDON_NAME")
-
-        # Detect existing key (could be "omsagent" or "omsAgent" from Azure API)
-        existing_key = None
-        if CONST_MONITORING_ADDON_NAME in mc.addon_profiles:
-            existing_key = CONST_MONITORING_ADDON_NAME
-        elif CONST_MONITORING_ADDON_NAME_CAMELCASE in mc.addon_profiles:
-            existing_key = CONST_MONITORING_ADDON_NAME_CAMELCASE
-
-        if existing_key:
-            addon_profile = mc.addon_profiles[existing_key]
-        else:
-            addon_profile = self.models.ManagedClusterAddonProfile(enabled=False)
-            existing_key = CONST_MONITORING_ADDON_NAME
-
-        addon_profile.enabled = True
+        CONST_MONITORING_LOG_ANALYTICS_WORKSPACE_RESOURCE_ID = addon_consts.get(
+            "CONST_MONITORING_LOG_ANALYTICS_WORKSPACE_RESOURCE_ID")
+        CONST_MONITORING_USING_AAD_MSI_AUTH = addon_consts.get("CONST_MONITORING_USING_AAD_MSI_AUTH")
 
         # Get or create workspace resource ID
         workspace_resource_id = self.context.raw_param.get("workspace_resource_id")
@@ -7628,16 +7616,30 @@ def _setup_azure_monitor_logs(self, mc: ManagedCluster) -> None:
         sanitize_func = self.context.external_functions.sanitize_loganalytics_ws_resource_id
         workspace_resource_id = sanitize_func(workspace_resource_id)
 
-        CONST_MONITORING_LOG_ANALYTICS_WORKSPACE_RESOURCE_ID = addon_consts.get(
-            "CONST_MONITORING_LOG_ANALYTICS_WORKSPACE_RESOURCE_ID")
-        CONST_MONITORING_USING_AAD_MSI_AUTH = addon_consts.get("CONST_MONITORING_USING_AAD_MSI_AUTH")
-
+        # Call get_enable_msi_auth_for_monitoring BEFORE detecting the existing key,
+        # because the parent's implementation may normalize addon_profiles keys in-place
+        # (e.g., renaming "omsAgent" to "omsagent").
         enable_msi_auth_bool = self.context.get_enable_msi_auth_for_monitoring()
         if enable_msi_auth_bool:
             enable_msi_auth = "true"
         else:
             enable_msi_auth = "false"
 
+        # Detect existing key (could be "omsagent" or "omsAgent" from Azure API)
+        existing_key = None
+        if CONST_MONITORING_ADDON_NAME in mc.addon_profiles:
+            existing_key = CONST_MONITORING_ADDON_NAME
+        elif CONST_MONITORING_ADDON_NAME_CAMELCASE in mc.addon_profiles:
+            existing_key = CONST_MONITORING_ADDON_NAME_CAMELCASE
+
+        if existing_key:
+            addon_profile = mc.addon_profiles[existing_key]
+        else:
+            addon_profile = self.models.ManagedClusterAddonProfile(enabled=False)
+            existing_key = CONST_MONITORING_ADDON_NAME
+
+        addon_profile.enabled = True
+
         new_config = {
             CONST_MONITORING_LOG_ANALYTICS_WORKSPACE_RESOURCE_ID: workspace_resource_id,
             CONST_MONITORING_USING_AAD_MSI_AUTH: enable_msi_auth
diff --git a/src/aks-preview/azext_aks_preview/tests/latest/test_managed_cluster_decorator.py b/src/aks-preview/azext_aks_preview/tests/latest/test_managed_cluster_decorator.py
index 3f0a16b3ff7..733eabcddd3 100644
--- a/src/aks-preview/azext_aks_preview/tests/latest/test_managed_cluster_decorator.py
+++ b/src/aks-preview/azext_aks_preview/tests/latest/test_managed_cluster_decorator.py
@@ -10922,12 +10922,13 @@ def test_setup_azure_monitor_logs_with_omsagent_camelcase(self):
         # Call _setup_azure_monitor_logs
         dec_1._setup_azure_monitor_logs(mc_1)
 
-        # Verify: Should update existing omsAgent key (not create omsagent lowercase)
-        self.assertIn("omsAgent", mc_1.addon_profiles)
-        self.assertNotIn("omsagent", mc_1.addon_profiles)  # Should NOT create duplicate
-        self.assertTrue(mc_1.addon_profiles["omsAgent"].enabled)
+        # Verify: The parent class normalizes addon keys to lowercase in-place,
+        # so "omsAgent" becomes "omsagent". The key point is no duplicate is created.
+        self.assertIn("omsagent", mc_1.addon_profiles)
+        self.assertEqual(len([k for k in mc_1.addon_profiles if k.lower() == "omsagent"]), 1)  # No duplicate
+        self.assertTrue(mc_1.addon_profiles["omsagent"].enabled)
         self.assertEqual(
-            mc_1.addon_profiles["omsAgent"].config["logAnalyticsWorkspaceResourceID"],
+            mc_1.addon_profiles["omsagent"].config["logAnalyticsWorkspaceResourceID"],
             "/subscriptions/test/resourceGroups/test/providers/Microsoft.OperationalInsights/workspaces/test-workspace"
         )