add `response_format` support for `completion` API (#9665)

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

@@ -108,6 +108,23 @@ class JsonSchemaResponseFormat(BaseModel):
     strict: Optional[bool] = False
 
 
+class ResponseFormat(BaseModel):
+    type: Literal["text", "json_object", "json_schema"]
+    json_schema: Optional[JsonSchemaResponseFormat] = None
+
+
+class StructuresResponseFormat(BaseModel):
+    begin: str
+    schema_: Optional[Dict[str, object]] = Field(alias="schema", default=None)
+    end: str
+
+
+class StructuralTagResponseFormat(BaseModel):
+    type: Literal["structural_tag"]
+    structures: List[StructuresResponseFormat]
+    triggers: List[str]
+
+
 class FileRequest(BaseModel):
     # https://platform.openai.com/docs/api-reference/files/create
     file: bytes  # The File object (not file name) to be uploaded
@@ -200,6 +217,7 @@ class CompletionRequest(BaseModel):
     skip_special_tokens: bool = True
     lora_path: Optional[Union[List[Optional[str]], Optional[str]]] = None
     session_params: Optional[Dict] = None
+    response_format: Optional[Union[ResponseFormat, StructuralTagResponseFormat]] = None
 
     # For PD disaggregation
     bootstrap_host: Optional[Union[List[str], str]] = None
@@ -359,23 +377,6 @@ ChatCompletionMessageParam = Union[
 ]
 
 
-class ResponseFormat(BaseModel):
-    type: Literal["text", "json_object", "json_schema"]
-    json_schema: Optional[JsonSchemaResponseFormat] = None
-
-
-class StructuresResponseFormat(BaseModel):
-    begin: str
-    schema_: Optional[Dict[str, object]] = Field(alias="schema", default=None)
-    end: str
-
-
-class StructuralTagResponseFormat(BaseModel):
-    type: Literal["structural_tag"]
-    structures: List[StructuresResponseFormat]
-    triggers: List[str]
-
-
 class Function(BaseModel):
     """Function descriptions."""
 

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

@@ -23,6 +23,7 @@ from sglang.srt.entrypoints.openai.utils import (
 from sglang.srt.managers.io_struct import GenerateReqInput
 from sglang.srt.managers.template_manager import TemplateManager
 from sglang.srt.managers.tokenizer_manager import TokenizerManager
+from sglang.utils import convert_json_schema_to_str
 
 logger = logging.getLogger(__name__)
 
@@ -125,6 +126,20 @@ class OpenAIServingCompletion(OpenAIServingBase):
             "logit_bias": request.logit_bias,
         }
 
+        # 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_
+            )
+        elif request.response_format and request.response_format.type == "json_object":
+            sampling_params["json_schema"] = '{"type": "object"}'
+        elif (
+            request.response_format and request.response_format.type == "structural_tag"
+        ):
+            sampling_params["structural_tag"] = convert_json_schema_to_str(
+                request.response_format.model_dump(by_alias=True)
+            )
+
         return sampling_params
 
     async def _handle_streaming_request(

test/srt/openai_server/basic/test_serving_completions.py

@@ -95,6 +95,63 @@ class ServingCompletionTestCase(unittest.TestCase):
         self.sc.tokenizer_manager.tokenizer.decode.return_value = "decoded"
         self.assertEqual(self.sc._prepare_echo_prompts(req), ["decoded"])
 
+    # ---------- response_format handling ----------
+    def test_response_format_json_object(self):
+        """Test that response_format json_object is correctly processed in sampling params."""
+        req = CompletionRequest(
+            model="x",
+            prompt="Generate a JSON object:",
+            max_tokens=100,
+            response_format={"type": "json_object"},
+        )
+        sampling_params = self.sc._build_sampling_params(req)
+        self.assertEqual(sampling_params["json_schema"], '{"type": "object"}')
+
+    def test_response_format_json_schema(self):
+        """Test that response_format json_schema is correctly processed in sampling params."""
+        schema = {
+            "type": "object",
+            "properties": {"name": {"type": "string"}, "age": {"type": "integer"}},
+        }
+        req = CompletionRequest(
+            model="x",
+            prompt="Generate a JSON object:",
+            max_tokens=100,
+            response_format={
+                "type": "json_schema",
+                "json_schema": {"name": "person", "schema": schema},
+            },
+        )
+        sampling_params = self.sc._build_sampling_params(req)
+        # The schema should be converted to string by convert_json_schema_to_str
+        self.assertIn("json_schema", sampling_params)
+        self.assertIsInstance(sampling_params["json_schema"], str)
+
+    def test_response_format_structural_tag(self):
+        """Test that response_format structural_tag is correctly processed in sampling params."""
+        req = CompletionRequest(
+            model="x",
+            prompt="Generate structured output:",
+            max_tokens=100,
+            response_format={
+                "type": "structural_tag",
+                "structures": [{"begin": "<data>", "end": "</data>"}],
+                "triggers": ["<data>"],
+            },
+        )
+        sampling_params = self.sc._build_sampling_params(req)
+        # The structural_tag should be processed
+        self.assertIn("structural_tag", sampling_params)
+        self.assertIsInstance(sampling_params["structural_tag"], str)
+
+    def test_response_format_none(self):
+        """Test that no response_format doesn't add extra constraints."""
+        req = CompletionRequest(model="x", prompt="Generate text:", max_tokens=100)
+        sampling_params = self.sc._build_sampling_params(req)
+        # Should not have json_schema or structural_tag from response_format
+        # (but might have json_schema from the legacy json_schema field)
+        self.assertIsNone(sampling_params.get("structural_tag"))
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=2)