fix(frontend): return 400 for missing completions json_schema (#28090)

Signed-off-by: Ting Sun <suntcrick@gmail.com>

python/sglang/srt/entrypoints/openai/serving_completions.py

@@ -164,9 +164,13 @@ class OpenAIServingCompletion(OpenAIServingBase):
 
         # Handle response_format constraints
         if request.response_format and request.response_format.type == "json_schema":
-            sampling_params["json_schema"] = convert_json_schema_to_str(
-                request.response_format.json_schema.schema_
-            )
+            json_schema = request.response_format.json_schema
+            schema = getattr(json_schema, "schema_", None)
+            if schema is None:
+                raise ValueError(
+                    "schema_ is required for json_schema response format request."
+                )
+            sampling_params["json_schema"] = convert_json_schema_to_str(schema)
         elif request.response_format and request.response_format.type == "json_object":
             sampling_params["json_schema"] = '{"type": "object"}'
         elif (

test/registered/unit/entrypoints/openai/test_serving_completions.py

@@ -140,6 +140,17 @@ class ServingCompletionTestCase(unittest.TestCase):
         self.assertIn("json_schema", sampling_params)
         self.assertIsInstance(sampling_params["json_schema"], str)
 
+    def test_response_format_json_schema_missing_schema(self):
+        """Test that json_schema response_format without a schema raises a ValueError."""
+        req = CompletionRequest(
+            model="x",
+            prompt="Generate a JSON object:",
+            max_tokens=100,
+            response_format={"type": "json_schema"},
+        )
+        with self.assertRaises(ValueError):
+            self.sc._build_sampling_params(req)
+
     def test_response_format_structural_tag(self):
         """Test that response_format structural_tag is correctly processed in sampling params."""
         req = CompletionRequest(