[Bugfix][Frontend] respect provided default guided decoding backend (#15476)

Signed-off-by: Guillaume Calmettes <gcalmettes@scaleway.com>

tests/test_sampling_params.py

 # SPDX-License-Identifier: Apache-2.0
 """Tests for the SamplingParams class.
 """
+
+import pytest
+
 from vllm import SamplingParams
+from vllm.config import ModelConfig
+from vllm.entrypoints.openai.protocol import ChatCompletionRequest
+
+MODEL_NAME = "Qwen/Qwen1.5-7B"
 
 
 def test_max_tokens_none():
@@ -9,6 +16,74 @@ def test_max_tokens_none():
     SamplingParams(temperature=0.01, top_p=0.1, max_tokens=None)
 
 
-if __name__ == "__main__":
-    import pytest
-    pytest.main([__file__])
+@pytest.fixture(scope="module")
+def model_config():
+    return ModelConfig(
+        MODEL_NAME,
+        task="auto",
+        tokenizer=MODEL_NAME,
+        tokenizer_mode="auto",
+        trust_remote_code=False,
+        seed=0,
+        dtype="float16",
+        revision=None,
+    )
+
+
+@pytest.fixture(scope="module")
+def default_max_tokens():
+    return 4096
+
+
+def test_sampling_params_from_request_with_no_guided_decoding_backend(
+        model_config, default_max_tokens):
+    # guided_decoding_backend is not present at request level
+    request = ChatCompletionRequest.model_validate({
+        'messages': [{
+            'role': 'user',
+            'content': 'Hello'
+        }],
+        'model':
+        MODEL_NAME,
+        'response_format': {
+            'type': 'json_object',
+        },
+    })
+
+    sampling_params = request.to_sampling_params(
+        default_max_tokens,
+        model_config.logits_processor_pattern,
+    )
+    # we do not expect any backend to be present and the default
+    # guided_decoding_backend at engine level will be used.
+    assert sampling_params.guided_decoding.backend is None
+
+
+@pytest.mark.parametrize("request_level_guided_decoding_backend,expected",
+                         [("xgrammar", "xgrammar"),
+                          ("lm-format-enforcer", "lm-format-enforcer"),
+                          ("outlines", "outlines")])
+def test_sampling_params_from_request_with_guided_decoding_backend(
+        request_level_guided_decoding_backend: str, expected: str,
+        model_config, default_max_tokens):
+
+    request = ChatCompletionRequest.model_validate({
+        'messages': [{
+            'role': 'user',
+            'content': 'Hello'
+        }],
+        'model':
+        MODEL_NAME,
+        'response_format': {
+            'type': 'json_object',
+        },
+        'guided_decoding_backend':
+        request_level_guided_decoding_backend,
+    })
+
+    sampling_params = request.to_sampling_params(
+        default_max_tokens,
+        model_config.logits_processor_pattern,
+    )
+    # backend correctly identified in resulting sampling_params
+    assert sampling_params.guided_decoding.backend == expected

vllm/entrypoints/openai/protocol.py

@@ -476,8 +476,6 @@ class ChatCompletionRequest(OpenAIBaseModel):
                 json_schema = self.response_format.json_schema
                 assert json_schema is not None
                 self.guided_json = json_schema.json_schema
-                if self.guided_decoding_backend is None:
-                    self.guided_decoding_backend = "xgrammar"
 
         guided_decoding = GuidedDecodingParams.from_optional(
             json=self._get_guided_json_from_tool() or self.guided_json,