[diffusion] fix: fix some sampling args passed via cli are omitted (#20630)

python/sglang/multimodal_gen/configs/sample/sampling_params.py

@@ -575,7 +575,9 @@ class SamplingParams:
         user_kwargs.pop("diffusers_kwargs", None)
         user_sampling_params = SamplingParams(*args, **user_kwargs)
         # TODO: refactor
-        sampling_params._merge_with_user_params(user_sampling_params)
+        sampling_params._merge_with_user_params(
+            user_sampling_params, explicit_fields=set(user_kwargs.keys())
+        )
         sampling_params._adjust(server_args)
 
         sampling_params._validate_with_pipeline_config(server_args.pipeline_config)
@@ -641,7 +643,7 @@ class SamplingParams:
         parser.add_argument(
             "--negative-prompt",
             type=str,
-            default=SamplingParams.negative_prompt,
+            default=None,
             help="Negative text prompt for generation",
         )
         parser.add_argument(
@@ -928,16 +930,29 @@ class SamplingParams:
         sampling_params_fields = {attr.name for attr in dataclasses.fields(cls)}
         args_attrs = set(vars(args).keys())
         attrs = sampling_params_fields & args_attrs
-        return {attr: getattr(args, attr) for attr in attrs if hasattr(args, attr)}
+        return {
+            attr: getattr(args, attr)
+            for attr in attrs
+            if hasattr(args, attr) and getattr(args, attr) is not None
+        }
 
     def output_file_path(self):
         if self.output_path is None:
             return None
         return os.path.join(self.output_path, self.output_file_name)
 
-    def _merge_with_user_params(self, user_params: "SamplingParams"):
+    def _merge_with_user_params(
+        self,
+        user_params: "SamplingParams",
+        explicit_fields: set[str] | None = None,
+    ):
         """
         Merges parameters from a user-provided SamplingParams object.
+
+        Args:
+            explicit_fields: field names explicitly set by the user (e.g. from
+                CLI kwargs). These are always treated as user-modified even when
+                their value matches the base-class default.
         """
         if user_params is None:
             return
@@ -951,8 +966,9 @@ class SamplingParams:
             user_value = getattr(user_params, field_name)
             default_class_value = getattr(SamplingParams, field_name)
 
-            # A field is considered user-modified if its value is different from the default
+            is_user_modified = user_value != default_class_value or (
-            is_user_modified = user_value != default_class_value
+                explicit_fields is not None and field_name in explicit_fields
+            )
             is_protected_field = field_name in predefined_fields
             if is_user_modified and (
                 allow_override_protected or not is_protected_field

python/sglang/multimodal_gen/test/unit/test_sampling_params.py

@@ -1,3 +1,4 @@
+import argparse
 import math
 import unittest
 
@@ -67,5 +68,82 @@ class TestSamplingParamsSubclass(unittest.TestCase):
             DiffusersGenericSamplingParams(num_frames=0)
 
 
+class TestNegativePromptMerge(unittest.TestCase):
+    """Regression tests for negative_prompt not being passed through CLI"""
+
+    def test_get_cli_args_filters_none(self):
+        ns = argparse.Namespace(negative_prompt=None, prompt="hello")
+        result = SamplingParams.get_cli_args(ns)
+        self.assertNotIn("negative_prompt", result)
+        self.assertEqual(result["prompt"], "hello")
+
+    def test_get_cli_args_keeps_explicit_value(self):
+        ns = argparse.Namespace(negative_prompt="ugly, blurry")
+        result = SamplingParams.get_cli_args(ns)
+        self.assertEqual(result["negative_prompt"], "ugly, blurry")
+
+    def test_merge_preserves_subclass_default_when_not_explicit(self):
+        """Without explicit_fields, value matching base default is not merged,
+        so the subclass default (empty string) is preserved."""
+        target = DiffusersGenericSamplingParams()
+        self.assertEqual(target.negative_prompt, "")
+
+        user = SamplingParams()
+        target._merge_with_user_params(user)
+        self.assertEqual(target.negative_prompt, "")
+
+    def test_merge_applies_different_negative_prompt(self):
+        target = DiffusersGenericSamplingParams()
+        user = SamplingParams(negative_prompt="ugly, blurry")
+        target._merge_with_user_params(user)
+        self.assertEqual(target.negative_prompt, "ugly, blurry")
+
+    def test_merge_explicit_field_matching_base_default(self):
+        """Even when the user value matches the base-class default, it should
+        still be applied if listed in explicit_fields."""
+        base_default = SamplingParams.negative_prompt
+        target = DiffusersGenericSamplingParams()
+        self.assertEqual(target.negative_prompt, "")
+
+        user = SamplingParams(negative_prompt=base_default)
+        target._merge_with_user_params(user, explicit_fields={"negative_prompt"})
+        self.assertEqual(target.negative_prompt, base_default)
+
+    def test_cli_roundtrip_no_negative_prompt(self):
+        """Simulate CLI without --negative-prompt: subclass default is kept."""
+        ns = argparse.Namespace(negative_prompt=None, width=512, height=512)
+        kwargs = SamplingParams.get_cli_args(ns)
+        self.assertNotIn("negative_prompt", kwargs)
+
+        user = SamplingParams(**kwargs)
+        target = DiffusersGenericSamplingParams()
+        target._merge_with_user_params(user, explicit_fields=set(kwargs.keys()))
+        self.assertEqual(target.negative_prompt, "")
+
+    def test_cli_roundtrip_with_negative_prompt(self):
+        """Simulate CLI with --negative-prompt: user value is applied."""
+        user_neg = "bad quality, watermark"
+        ns = argparse.Namespace(negative_prompt=user_neg, width=512, height=512)
+        kwargs = SamplingParams.get_cli_args(ns)
+        user = SamplingParams(**kwargs)
+
+        target = DiffusersGenericSamplingParams()
+        target._merge_with_user_params(user, explicit_fields=set(kwargs.keys()))
+        self.assertEqual(target.negative_prompt, user_neg)
+
+    def test_cli_roundtrip_with_base_default_negative_prompt(self):
+        """Simulate CLI where --negative-prompt value matches the base default:
+        user value should still be applied (not dropped)."""
+        base_default = SamplingParams.negative_prompt
+        ns = argparse.Namespace(negative_prompt=base_default, width=512, height=512)
+        kwargs = SamplingParams.get_cli_args(ns)
+        self.assertIn("negative_prompt", kwargs)
+
+        user = SamplingParams(**kwargs)
+        target = DiffusersGenericSamplingParams()
+        target._merge_with_user_params(user, explicit_fields=set(kwargs.keys()))
+        self.assertEqual(target.negative_prompt, base_default)
+
+
 if __name__ == "__main__":
     unittest.main()