[Frontend] Enable generic structured_outputs for responses API (#33709)

Signed-off-by: Alec Solder <alecs@fb.com>
Co-authored-by: Alec Solder <alecs@fb.com>

vllm/entrypoints/openai/responses/protocol.py

@@ -233,6 +233,10 @@ class ResponsesRequest(OpenAIBaseModel):
     # this cannot be used in conjunction with previous_response_id
     # TODO: consider supporting non harmony messages as well
     previous_input_messages: list[OpenAIHarmonyMessage | dict] | None = None
+    structured_outputs: StructuredOutputsParams | None = Field(
+        default=None,
+        description="Additional kwargs for structured outputs",
+    )
 
     repetition_penalty: float | None = None
     seed: int | None = Field(None, ge=_LONG_INFO.min, le=_LONG_INFO.max)
@@ -319,8 +323,14 @@ class ResponsesRequest(OpenAIBaseModel):
         stop_token_ids = default_sampling_params.get("stop_token_ids")
 
         # Structured output
-        structured_outputs = None
+        structured_outputs = self.structured_outputs
+
+        # Also check text.format for OpenAI-style json_schema
         if self.text is not None and self.text.format is not None:
+            if structured_outputs is not None:
+                raise ValueError(
+                    "Cannot specify both structured_outputs and text.format"
+                )
             response_format = self.text.format
             if (
                 response_format.type == "json_schema"
@@ -329,8 +339,6 @@ class ResponsesRequest(OpenAIBaseModel):
                 structured_outputs = StructuredOutputsParams(
                     json=response_format.schema_
                 )
-            elif response_format.type == "json_object":
-                raise NotImplementedError("json_object is not supported")
 
         stop = self.stop if self.stop else []
         if isinstance(stop, str):