diff --git a/tests/entrypoints/conftest.py b/tests/entrypoints/conftest.py
index 0f7d15e1d85aa..ef74062ce4b41 100644
--- a/tests/entrypoints/conftest.py
+++ b/tests/entrypoints/conftest.py
@@ -100,6 +100,45 @@ def sample_complex_json_schema():
     }
 
 
+@pytest.fixture
+def sample_definition_json_schema():
+    return {
+        '$defs': {
+            'Step': {
+                'properties': {
+                    'explanation': {
+                        'title': 'Explanation',
+                        'type': 'string'
+                    },
+                    'output': {
+                        'title': 'Output',
+                        'type': 'string'
+                    }
+                },
+                'required': ['explanation', 'output'],
+                'title': 'Step',
+                'type': 'object'
+            }
+        },
+        'properties': {
+            'steps': {
+                'items': {
+                    '$ref': '#/$defs/Step'
+                },
+                'title': 'Steps',
+                'type': 'array'
+            },
+            'final_answer': {
+                'title': 'Final Answer',
+                'type': 'string'
+            }
+        },
+        'required': ['steps', 'final_answer'],
+        'title': 'MathReasoning',
+        'type': 'object'
+    }
+
+
 @pytest.fixture
 def sample_guided_choice():
     return [
diff --git a/tests/entrypoints/llm/test_guided_generate.py b/tests/entrypoints/llm/test_guided_generate.py
index de6257cfc551c..ed50ec6bbc9eb 100644
--- a/tests/entrypoints/llm/test_guided_generate.py
+++ b/tests/entrypoints/llm/test_guided_generate.py
@@ -104,6 +104,34 @@ def test_guided_complex_json_completion(sample_complex_json_schema, llm):
                             schema=sample_complex_json_schema)
 
 
+@pytest.mark.skip_global_cleanup
+def test_guided_definition_json_completion(sample_definition_json_schema, llm):
+    sampling_params = SamplingParams(temperature=1.0,
+                                     max_tokens=1000,
+                                     guided_decoding=GuidedDecodingParams(
+                                         json=sample_definition_json_schema))
+    outputs = llm.generate(prompts=[
+        f"Give an example JSON for solving 8x + 7 = -23 "
+        f"that fits this schema: {sample_definition_json_schema}"
+    ] * 2,
+                           sampling_params=sampling_params,
+                           use_tqdm=True)
+
+    assert outputs is not None
+
+    for output in outputs:
+        assert output is not None
+        assert isinstance(output, RequestOutput)
+        prompt = output.prompt
+
+        generated_text = output.outputs[0].text
+        assert generated_text is not None
+        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+        output_json = json.loads(generated_text)
+        jsonschema.validate(instance=output_json,
+                            schema=sample_definition_json_schema)
+
+
 @pytest.mark.skip_global_cleanup
 def test_guided_choice_completion(sample_guided_choice, llm):
     sampling_params = SamplingParams(
diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index 6ed7c2e9dcd6b..5a70e0952666b 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -387,7 +387,7 @@ def to_sampling_params(
                 assert json_schema is not None
                 self.guided_json = json_schema.json_schema
                 if self.guided_decoding_backend is None:
-                    self.guided_decoding_backend = "lm-format-enforcer"
+                    self.guided_decoding_backend = "xgrammar"
 
         guided_decoding = GuidedDecodingParams.from_optional(
             json=self._get_guided_json_from_tool() or self.guided_json,