diff --git a/config-merger/script/merge_config.py b/config-merger/script/merge_config.py
index 5ae1c92..8c1c07d 100755
--- a/config-merger/script/merge_config.py
+++ b/config-merger/script/merge_config.py
@@ -26,7 +26,6 @@
 """
 
 import os
-import shutil
 import sys
 
 import yaml
@@ -383,11 +382,8 @@ def ensure_node_id(user_config_path):
             user_config['network'] = {}
         user_config['network']['node_id'] = node_id
 
-        # Write back to user config (atomic)
-        temp_path = user_config_path + '.node_id_tmp.' + str(os.getpid())
-        with open(temp_path, 'w') as f:
-            yaml.dump(user_config, f, default_flow_style=False, sort_keys=False)
-        os.rename(temp_path, user_config_path)
+        write_file_atomic(user_config_path, yaml.dump(
+            user_config, default_flow_style=False, sort_keys=False))
 
         print(f"Node ID set in {user_config_path}")
 
@@ -426,17 +422,9 @@ def main():
             print("User config not found, copying from defaults...")
             os.makedirs(os.path.dirname(user_config_path), exist_ok=True)
 
-            # Write to temp file first, then atomic rename
-            temp_path = user_config_path + '.tmp.' + str(os.getpid())
-            shutil.copy(default_config, temp_path)
-            try:
-                os.rename(temp_path, user_config_path)  # Atomic on POSIX
-                print(f"Created {user_config_path}")
-            except (FileExistsError, OSError):
-                # Another process created it first, clean up temp
-                if os.path.exists(temp_path):
-                    os.remove(temp_path)
-                print("User config was created by another process")
+            with open(default_config) as f:
+                write_file_atomic(user_config_path, f.read())
+            print(f"Created {user_config_path}")
 
         # Ensure node_id exists and matches hardware (add/update if needed, Pi only)
         ensure_node_id(user_config_path)
@@ -473,8 +461,8 @@ def main():
         # Write merged config to output
         print(f"Writing merged config to {output_config_path}")
         os.makedirs(os.path.dirname(output_config_path), exist_ok=True)
-        with open(output_config_path, 'w') as f:
-            yaml.dump(config, f, default_flow_style=False, sort_keys=False)
+        write_file_atomic(output_config_path, yaml.dump(
+            config, default_flow_style=False, sort_keys=False))
 
         # Generate .env file for tar1090-node
         generate_env_file(config, os.path.dirname(output_config_path))
diff --git a/config-merger/test/test_merge_config.py b/config-merger/test/test_merge_config.py
index 68f1070..0138a79 100755
--- a/config-merger/test/test_merge_config.py
+++ b/config-merger/test/test_merge_config.py
@@ -63,6 +63,14 @@ def run_merge(self):
 
         return output_yml
 
+    def test_config_yml_is_replaced_not_rewritten(self):
+        self.write_yaml(os.path.join(self.defaults_dir, 'default.yml'), {'network': {'ip': '0.0.0.0'}})
+        output = self.run_merge()
+        before = os.stat(output).st_ino
+        self.run_merge()
+        self.assertNotEqual(os.stat(output).st_ino, before)
+        self.assertEqual(self.read_yaml(output)['network']['ip'], '0.0.0.0')
+
     def test_defaults_only(self):
         """Test merge with only default.yml"""
         default_config = {
@@ -1061,6 +1069,26 @@ def test_env_reaches_both_compose_slots_as_new_files(self):
                 self.assertEqual(f.read(), expected)
         self.assertNotIn('\x00', expected)
 
+    def test_every_write_goes_through_write_file_atomic(self):
+        # ret9573ecda, 2026-08-27 23:23:18: config.yml, tar1090.env,
+        # retina-tracker.yaml and .env were all zeroed by one power cut.
+        # Temp-file-and-rename without fsync did not save the two written
+        # that way. The only write-mode open() allowed is the helper's own.
+        import ast
+        script = os.path.join(os.path.dirname(os.path.dirname(__file__)), 'script', 'merge_config.py')
+        with open(script) as f:
+            tree = ast.parse(f.read())
+        writers = []
+        for func in [n for n in ast.walk(tree) if isinstance(n, ast.FunctionDef)]:
+            for call in [n for n in ast.walk(func) if isinstance(n, ast.Call)]:
+                name = getattr(call.func, 'id', None) or getattr(call.func, 'attr', None)
+                mode = call.args[1].value if len(call.args) > 1 and isinstance(call.args[1], ast.Constant) else 'r'
+                if name == 'open' and any(c in str(mode) for c in 'wax'):
+                    writers.append(func.name)
+                if name in ('copy', 'copyfile', 'rename'):
+                    writers.append(func.name)
+        self.assertEqual(sorted(set(writers)), ['write_file_atomic'])
+
     def test_skips_a_slot_that_does_not_exist(self):
         root = os.path.join(self.test_dir, 'mender-docker-compose')
         os.makedirs(os.path.join(root, 'current', 'manifests'))