[diffusion] fix: preserve _explicit_fields across dataclasses.replace in DiffGenerator (#25308)

python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py

@@ -222,6 +222,12 @@ class DiffGenerator:
                 output_file_name=user_output_file_name,
                 image_path=image_paths_per_prompt[i],
             )
+            # `dataclasses.replace` drops non-field attrs; restore
+            # `_explicit_fields` so InputValidationStage honors user-supplied
+            # width/height, and mark the keys overridden above as explicit.
+            sampling_params._explicit_fields = getattr(
+                sampling_params_orig, "_explicit_fields", set()
+            ) | {"prompt", "output_file_name", "image_path"}
             sampling_params._set_output_file_name()
             req = prepare_request(
                 server_args=self.server_args,

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

@@ -279,6 +279,51 @@ class TestSamplingParamsCliArgs(unittest.TestCase):
         self.assertIn("width", explicit_fields)
         self.assertIn("height", explicit_fields)
 
+    def test_dataclasses_replace_preserves_explicit_fields(self):
+        """`dataclasses.replace` drops `_explicit_fields`; DiffGenerator must restore it."""
+        import dataclasses
+
+        server_args = MagicMock()
+        server_args.backend = "sglang"
+        server_args.model_id = None
+        server_args.pipeline_config = MagicMock()
+
+        with patch.object(
+            SamplingParams,
+            "from_pretrained",
+            side_effect=lambda *args, **kwargs: Flux2SamplingParams(),
+        ):
+            sampling_params_orig = SamplingParams.from_user_sampling_params_args(
+                "dummy-model",
+                server_args=server_args,
+                prompt="orig",
+                image_path="/tmp/in.png",
+                width=768,
+                height=512,
+            )
+
+        self.assertIn("width", sampling_params_orig._explicit_fields)
+        self.assertIn("height", sampling_params_orig._explicit_fields)
+
+        cloned = dataclasses.replace(
+            sampling_params_orig,
+            prompt="new",
+            output_file_name=None,
+            image_path="/tmp/in2.png",
+        )
+        self.assertFalse(hasattr(cloned, "_explicit_fields"))
+
+        # Mirror the restore done in DiffGenerator.generate().
+        cloned._explicit_fields = getattr(
+            sampling_params_orig, "_explicit_fields", set()
+        ) | {"prompt", "output_file_name", "image_path"}
+
+        explicit = set(cloned.build_request_extra()["explicit_fields"])
+        self.assertIn("width", explicit)
+        self.assertIn("height", explicit)
+        self.assertIn("prompt", explicit)
+        self.assertIn("image_path", explicit)
+
 
 if __name__ == "__main__":
     unittest.main()