diff --git a/src/scripts/write_variable.py b/src/scripts/write_variable.py
index d77862a..f014bf1 100644
--- a/src/scripts/write_variable.py
+++ b/src/scripts/write_variable.py
@@ -13,50 +13,42 @@ try:
     online_app, target_app = ensure_online_connection(primary_project)
     app_name = getattr(target_app, 'get_name', lambda: "Unknown")()
 
-    # The write counterpart of read_value() has shifted across CODESYS SPs.
-    # Some expose 'write_value(name, value)', some 'set_value(...)',
-    # and some only the batch form 'write_values([(name, value), ...])'.
-    # Probe each available method, then fall back to dumping the available
-    # methods on online_app so future debug sessions know what's exposed.
-    candidates = []
-
-    # Single-write methods, (name, value) style
-    for method_name in ('write_value', 'set_value', 'write', 'set'):
-        if hasattr(online_app, method_name):
-            candidates.append((method_name + '(name, value)', method_name, [(VARIABLE_PATH, VARIABLE_VALUE)]))
-
-    # Batch-write methods, [(name, value), ...] style
-    for method_name in ('write_values', 'set_values'):
-        if hasattr(online_app, method_name):
-            candidates.append((method_name + '([(name, value)])', method_name, [[(VARIABLE_PATH, VARIABLE_VALUE)]]))
-
-    if not candidates:
-        # No known method exposed. Dump diagnostic.
-        attrs = sorted([a for a in dir(online_app) if not a.startswith('_')])
-        raise TypeError(
-            "Online application exposes no known write method "
-            "(tried write_value/set_value/write/set/write_values/set_values).\n"
-            "Available attributes: %s" % attrs
-        )
-
+    # SP21+/SP22 uses a two-step prepare-then-write pattern:
+    #   1) set_prepared_value(name, value) -- stage the value
+    #   2) write_prepared_values()         -- commit the staged writes
+    # Older SPs (and some sandboxed configurations) expose direct
+    # write_value(name, value) / write(name, value). Try the modern path
+    # first, fall back to direct.
     written = False
     last_err = None
-    for desc, method_name, args_list in candidates:
+
+    if hasattr(online_app, 'set_prepared_value') and hasattr(online_app, 'write_prepared_values'):
         try:
-            getattr(online_app, method_name)(*args_list)
-            print("DEBUG: %s OK" % desc)
+            online_app.set_prepared_value(VARIABLE_PATH, VARIABLE_VALUE)
+            online_app.write_prepared_values()
+            print("DEBUG: set_prepared_value + write_prepared_values OK")
             written = True
-            break
         except Exception as e:
             last_err = e
-            print("DEBUG: %s failed: %s: %s" % (desc, type(e).__name__, e))
+            print("DEBUG: set_prepared_value + write_prepared_values failed: %s: %s" % (type(e).__name__, e))
+
+    if not written:
+        for method_name in ('write_value', 'set_value', 'write', 'set'):
+            if not hasattr(online_app, method_name):
+                continue
+            try:
+                getattr(online_app, method_name)(VARIABLE_PATH, VARIABLE_VALUE)
+                print("DEBUG: %s(name, value) OK" % method_name)
+                written = True
+                break
+            except Exception as e:
+                last_err = e
+                print("DEBUG: %s(name, value) failed: %s: %s" % (method_name, type(e).__name__, e))
 
     if not written:
-        # All known method names were exposed but every call failed. Dump
-        # diagnostic + last error so we can iterate.
         attrs = sorted([a for a in dir(online_app) if not a.startswith('_')])
         raise RuntimeError(
-            "All known write methods were rejected. Last error: %s\n"
+            "Could not write variable. Last error: %s\n"
             "Available attributes on online_app: %s" % (last_err, attrs)
         )