[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>

tests/entrypoints/openai/responses/test_sampling_params.py

@@ -4,8 +4,17 @@
 """Unit tests for ResponsesRequest.to_sampling_params() parameter mapping."""
 
 import pytest
+import torch
+from openai.types.responses.response_format_text_json_schema_config import (
+    ResponseFormatTextJSONSchemaConfig,
+)
+from pydantic import ValidationError
 
-from vllm.entrypoints.openai.responses.protocol import ResponsesRequest
+from vllm.entrypoints.openai.responses.protocol import (
+    ResponsesRequest,
+    ResponseTextConfig,
+)
+from vllm.sampling_params import StructuredOutputsParams
 
 
 class TestResponsesRequestSamplingParams:
@@ -76,9 +85,6 @@ class TestResponsesRequestSamplingParams:
 
     def test_seed_bounds_validation(self):
         """Test that seed values outside torch.long bounds are rejected."""
-        import torch
-        from pydantic import ValidationError
-
         # Test seed below minimum
         with pytest.raises(ValidationError) as exc_info:
             ResponsesRequest(
@@ -111,3 +117,40 @@ class TestResponsesRequestSamplingParams:
             seed=torch.iinfo(torch.long).max,
         )
         assert request_max.seed == torch.iinfo(torch.long).max
+
+    def test_structured_outputs_passed_through(self):
+        """Test that structured_outputs field is passed to SamplingParams."""
+        structured_outputs = StructuredOutputsParams(grammar="root ::= 'hello'")
+        request = ResponsesRequest(
+            model="test-model",
+            input="test input",
+            structured_outputs=structured_outputs,
+        )
+
+        sampling_params = request.to_sampling_params(default_max_tokens=1000)
+
+        assert sampling_params.structured_outputs is not None
+        assert sampling_params.structured_outputs.grammar == "root ::= 'hello'"
+
+    def test_structured_outputs_and_json_schema_conflict(self):
+        """Test that specifying both structured_outputs and json_schema raises."""
+        structured_outputs = StructuredOutputsParams(grammar="root ::= 'hello'")
+        text_config = ResponseTextConfig()
+        text_config.format = ResponseFormatTextJSONSchemaConfig(
+            type="json_schema",
+            name="test",
+            schema={"type": "object"},
+        )
+        request = ResponsesRequest(
+            model="test-model",
+            input="test input",
+            structured_outputs=structured_outputs,
+            text=text_config,
+        )
+
+        with pytest.raises(ValueError) as exc_info:
+            request.to_sampling_params(default_max_tokens=1000)
+
+        assert "Cannot specify both structured_outputs and text.format" in str(
+            exc_info.value
+        )