diff --git a/python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py b/python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py
index 2f71ad1d1..6e21f13b4 100644
--- a/python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py
+++ b/python/sglang/multimodal_gen/runtime/entrypoints/diffusion_generator.py
@@ -183,7 +183,10 @@ class DiffGenerator:
         multiple prompts, or None when every request failed.
         """
         # 1. prepare requests
-        prompts = self._resolve_prompts(sampling_params_kwargs.get("prompt"))
+        prompts = self._resolve_prompts(
+            sampling_params_kwargs.get("prompt"),
+            sampling_params_kwargs.get("prompt_path"),
+        )
         user_output_file_name = sampling_params_kwargs.get("output_file_name")
 
         if len(prompts) > 1 and user_output_file_name is not None:
@@ -334,10 +337,14 @@ class DiffGenerator:
             return None
         return results[0] if len(results) == 1 else results
 
-    def _resolve_prompts(self, prompt: str | list[str] | None) -> list[str]:
+    def _resolve_prompts(
+        self,
+        prompt: str | list[str] | None,
+        prompt_path: str | None = None,
+    ) -> list[str]:
         """Collect prompts from the argument or from a prompt file."""
-        if self.server_args.prompt_file_path is not None:
-            path = self.server_args.prompt_file_path
+        path = prompt_path or self.server_args.prompt_file_path
+        if path is not None:
             if not os.path.exists(path):
                 raise FileNotFoundError(f"Prompt text file not found: {path}")
             with open(path, encoding="utf-8") as f:
diff --git a/python/sglang/multimodal_gen/test/run_suite.py b/python/sglang/multimodal_gen/test/run_suite.py
index 7865f96f9..e0f598f7c 100644
--- a/python/sglang/multimodal_gen/test/run_suite.py
+++ b/python/sglang/multimodal_gen/test/run_suite.py
@@ -36,6 +36,7 @@ SUITES = {
         "../unit/test_lora_format_adapter.py",
         "../unit/test_server_args.py",
         "../unit/test_input_validation.py",
+        "../unit/test_resolve_prompts.py",
         # add new unit tests here
     ],
     "1-gpu": [
diff --git a/python/sglang/multimodal_gen/test/unit/test_resolve_prompts.py b/python/sglang/multimodal_gen/test/unit/test_resolve_prompts.py
new file mode 100644
index 000000000..73e5cdb9e
--- /dev/null
+++ b/python/sglang/multimodal_gen/test/unit/test_resolve_prompts.py
@@ -0,0 +1,99 @@
+import os
+import tempfile
+import unittest
+from types import SimpleNamespace
+
+from sglang.multimodal_gen.runtime.entrypoints.diffusion_generator import DiffGenerator
+
+
+def _make_generator(prompt_file_path=None):
+    """Return a DiffGenerator-shaped object with only server_args populated."""
+    obj = object.__new__(DiffGenerator)
+    obj.server_args = SimpleNamespace(prompt_file_path=prompt_file_path)
+    return obj
+
+
+class TestResolvePrompts(unittest.TestCase):
+    # ---- inline prompt ----
+    def test_none_prompt_returns_space(self):
+        gen = _make_generator()
+        self.assertEqual(gen._resolve_prompts(None), [" "])
+
+    def test_string_prompt(self):
+        gen = _make_generator()
+        self.assertEqual(gen._resolve_prompts("hello"), ["hello"])
+
+    def test_list_prompt(self):
+        gen = _make_generator()
+        self.assertEqual(gen._resolve_prompts(["a", "b"]), ["a", "b"])
+
+    # ---- prompt_path (SamplingParams) ----
+    def test_prompt_path_single_line(self):
+        gen = _make_generator()
+        with tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False) as f:
+            f.write("sunset over the ocean\n")
+            path = f.name
+        try:
+            result = gen._resolve_prompts(None, prompt_path=path)
+            self.assertEqual(result, ["sunset over the ocean"])
+        finally:
+            os.unlink(path)
+
+    def test_prompt_path_multi_line(self):
+        gen = _make_generator()
+        with tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False) as f:
+            f.write("line one\n\nline two\n")
+            path = f.name
+        try:
+            result = gen._resolve_prompts(None, prompt_path=path)
+            self.assertEqual(result, ["line one", "line two"])
+        finally:
+            os.unlink(path)
+
+    def test_prompt_path_takes_priority_over_server_args(self):
+        with tempfile.NamedTemporaryFile(
+            "w", suffix=".txt", delete=False
+        ) as f1, tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False) as f2:
+            f1.write("from prompt_path\n")
+            f2.write("from server_args\n")
+            path1, path2 = f1.name, f2.name
+        try:
+            gen = _make_generator(prompt_file_path=path2)
+            result = gen._resolve_prompts(None, prompt_path=path1)
+            self.assertEqual(result, ["from prompt_path"])
+        finally:
+            os.unlink(path1)
+            os.unlink(path2)
+
+    # ---- prompt_file_path (ServerArgs) ----
+    def test_server_args_prompt_file_path(self):
+        with tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False) as f:
+            f.write("from server args\n")
+            path = f.name
+        try:
+            gen = _make_generator(prompt_file_path=path)
+            result = gen._resolve_prompts(None)
+            self.assertEqual(result, ["from server args"])
+        finally:
+            os.unlink(path)
+
+    # ---- error cases ----
+    def test_missing_file_raises(self):
+        gen = _make_generator()
+        with self.assertRaises(FileNotFoundError):
+            gen._resolve_prompts(None, prompt_path="/nonexistent/file.txt")
+
+    def test_empty_file_raises(self):
+        with tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False) as f:
+            f.write("   \n\n  \n")
+            path = f.name
+        try:
+            gen = _make_generator()
+            with self.assertRaises(ValueError):
+                gen._resolve_prompts(None, prompt_path=path)
+        finally:
+            os.unlink(path)
+
+
+if __name__ == "__main__":
+    unittest.main()