From 2bc469a9ffb3a85a5af0f1294f2fb646d05a6952 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 20 Jul 2026 14:55:37 +0000
Subject: [PATCH 1/2] Initial plan


From 815239d16d0d13757dcd7201d3fcefc5673b4e7b Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Mon, 20 Jul 2026 15:04:06 +0000
Subject: [PATCH 2/2] [VM] fix `az vmss extension set` not updating existing
 extension

---
 .../azure/cli/command_modules/vm/custom.py    |  2 +-
 .../tests/latest/test_custom_vm_commands.py   | 60 +++++++++++++++++++
 2 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/src/azure-cli/azure/cli/command_modules/vm/custom.py b/src/azure-cli/azure/cli/command_modules/vm/custom.py
index 0dcef36551a..5f4633ef42d 100644
--- a/src/azure-cli/azure/cli/command_modules/vm/custom.py
+++ b/src/azure-cli/azure/cli/command_modules/vm/custom.py
@@ -5328,7 +5328,7 @@ def set_vmss_extension(cmd, resource_group_name, vmss_name, extension_name, publ
         if extensions:
             extension_profile['extensions'] = \
                 [x for x in extensions if
-                 x.get('type_properties_type', '').lower() != extension_name.lower() or
+                 x.get('type', '').lower() != extension_name.lower() or
                  x.get('publisher', '').lower() != publisher.lower()]
 
     ext = {
diff --git a/src/azure-cli/azure/cli/command_modules/vm/tests/latest/test_custom_vm_commands.py b/src/azure-cli/azure/cli/command_modules/vm/tests/latest/test_custom_vm_commands.py
index 0291e22490b..1a2ddd7bc93 100644
--- a/src/azure-cli/azure/cli/command_modules/vm/tests/latest/test_custom_vm_commands.py
+++ b/src/azure-cli/azure/cli/command_modules/vm/tests/latest/test_custom_vm_commands.py
@@ -196,6 +196,66 @@ class ErrorToExitCommandEarly(Exception):
             get_sdk_mock.assert_called_with(cli_ctx_mock, ResourceType.DATA_STORAGE_BLOB, '_blob_client#BlobClient')
 
 
+class TestSetVmssExtension(unittest.TestCase):
+
+    @mock.patch('azure.cli.command_modules.vm.custom.get_vmss_by_aaz')
+    @mock.patch('azure.cli.command_modules.vm.custom._normalize_extension_version')
+    @mock.patch('azure.cli.command_modules.vm.operations.vmss.VMSSCreate')
+    def test_set_vmss_extension_updates_existing(self, mock_vmss_create, mock_normalize_version,
+                                                 mock_get_vmss):
+        """Test that set_vmss_extension replaces an existing extension instead of duplicating it."""
+        from azure.cli.command_modules.vm.custom import set_vmss_extension
+
+        mock_normalize_version.return_value = '2.1'
+        mock_vmss_create_instance = mock.MagicMock()
+        mock_vmss_create.return_value = mock_vmss_create_instance
+
+        # Simulate a VMSS with an existing CustomScript extension (as returned by get_vmss_by_aaz)
+        mock_get_vmss.return_value = {
+            'location': 'eastus',
+            'virtualMachineProfile': {
+                'storageProfile': {'imageReference': {}},
+                'extensionProfile': {
+                    'extensions': [
+                        {
+                            'name': 'myScript',
+                            'type': 'CustomScript',
+                            'publisher': 'Microsoft.Azure.Extensions',
+                            'typeHandlerVersion': '2.0',
+                            'autoUpgradeMinorVersion': True,
+                            'settings': {'commandToExecute': 'echo old'},
+                        }
+                    ]
+                }
+            }
+        }
+
+        cmd = mock.MagicMock()
+        cmd.cli_ctx = DummyCli()
+
+        set_vmss_extension(
+            cmd,
+            resource_group_name='myRG',
+            vmss_name='myVMSS',
+            extension_name='CustomScript',
+            publisher='Microsoft.Azure.Extensions',
+            version='2.1',
+            settings={'commandToExecute': 'echo new'},
+            extension_instance_name='myScript',
+        )
+
+        # Verify VMSSCreate was called with the updated extension list
+        call_args = mock_vmss_create_instance.call_args
+        command_args = call_args[1]['command_args']
+        extensions = command_args['virtual_machine_profile']['extension_profile']['extensions']
+
+        # There should be exactly one extension (the old one replaced, not duplicated)
+        self.assertEqual(len(extensions), 1)
+        self.assertEqual(extensions[0]['name'], 'myScript')
+        self.assertEqual(extensions[0]['type'], 'CustomScript')
+        self.assertEqual(extensions[0]['settings'], {'commandToExecute': 'echo new'})
+
+
 class FakedVM:  # pylint: disable=too-few-public-methods
     def __init__(self, nics=None, disks=None, os_disk=None):
         self.network_profile = NetworkProfile(network_interfaces=nics)