Fix: Respect required=False in add_lightning_class_args when subclass_mode=False

src/lightning/pytorch/cli.py

@@ -130,7 +130,7 @@ def add_lightning_class_args(
         Args:
             lightning_class: A callable or any subclass of {Trainer, LightningModule, LightningDataModule, Callback}.
             nested_key: Name of the nested namespace to store arguments.
-            subclass_mode: Whether allow any subclass of the given class.
+            subclass_mode: Whether to allow any subclass of the given class.
             required: Whether the argument group is required.
 
         Returns:
@@ -145,15 +145,26 @@ def add_lightning_class_args(
         ):
             if issubclass(lightning_class, Callback):
                 self.callback_keys.append(nested_key)
+
+            # NEW LOGIC: If subclass_mode=False and required=False, only add if config provides this key
+            if not subclass_mode and not required:
+                config_path = f"{self.subcommand}.{nested_key}" if getattr(self, "subcommand", None) else nested_key
+                config = getattr(self, "config", {})
+                if not any(k.startswith(config_path) for k in config):
+                    # Skip adding class arguments
+                    return []
+
             if subclass_mode:
                 return self.add_subclass_arguments(lightning_class, nested_key, fail_untyped=False, required=required)
+
             return self.add_class_arguments(
                 lightning_class,
                 nested_key,
                 fail_untyped=False,
                 instantiate=not issubclass(lightning_class, Trainer),
                 sub_configs=True,
             )
+
         raise MisconfigurationException(
             f"Cannot add arguments from: {lightning_class}. You should provide either a callable or a subclass of: "
             "Trainer, LightningModule, LightningDataModule, or Callback."

tests/tests_pytorch/test_cli.py

@@ -1789,3 +1789,31 @@ def test_lightning_cli_with_args_given(args):
 def test_lightning_cli_args_and_sys_argv_warning():
     with mock.patch("sys.argv", ["", "--model.foo=456"]), pytest.warns(Warning, match="LightningCLI's args parameter "):
         LightningCLI(TestModel, run=False, args=["--model.foo=789"])
+
+
+def test_add_class_args_required_false_skips_addition(tmp_path):
+    from lightning.pytorch import callbacks, cli
+
+    class FooCheckpoint(callbacks.ModelCheckpoint):
+        def __init__(self, dirpath, *args, **kwargs):
+            super().__init__(dirpath, *args, **kwargs)
+
+    class SimpleModel:
+        def __init__(self):
+            pass
+
+    class SimpleDataModule:
+        def __init__(self):
+            pass
+
+    class FooCLI(cli.LightningCLI):
+        def __init__(self):
+            super().__init__(
+                model_class=SimpleModel, datamodule_class=SimpleDataModule, run=False, save_config_callback=None
+            )
+
+        def add_arguments_to_parser(self, parser):
+            parser.add_lightning_class_args(FooCheckpoint, "checkpoint", required=False)
+
+    # Expectation: No error raised even though FooCheckpoint requires `dirpath`
+    FooCLI()