Add outlines fallback when JSON schema has enum (#13449)

Signed-off-by: mgoin <mgoin64@gmail.com>

tests/entrypoints/conftest.py

@@ -141,6 +141,47 @@ def sample_definition_json_schema():
     }
 
 
+@pytest.fixture
+def sample_enum_json_schema():
+    return {
+        "type": "object",
+        "properties": {
+            "status": {
+                "type": "string",
+                "enum": ["active", "inactive",
+                         "pending"]  # Literal values using enum
+            },
+            "priority": {
+                "type": "string",
+                "enum": ["low", "medium", "high", "critical"]
+            },
+            "category": {
+                "type": "object",
+                "properties": {
+                    "type": {
+                        "type": "string",
+                        "enum": ["bug", "feature", "improvement"]
+                    },
+                    "severity": {
+                        "type": "integer",
+                        "enum": [1, 2, 3, 4,
+                                 5]  # Enum can also contain numbers
+                    }
+                },
+                "required": ["type", "severity"]
+            },
+            "flags": {
+                "type": "array",
+                "items": {
+                    "type": "string",
+                    "enum": ["urgent", "blocked", "needs_review", "approved"]
+                }
+            }
+        },
+        "required": ["status", "priority", "category", "flags"]
+    }
+
+
 @pytest.fixture
 def sample_guided_choice():
     return [

tests/entrypoints/llm/test_guided_generate.py

@@ -146,6 +146,47 @@ def test_guided_definition_json_completion(sample_definition_json_schema, llm,
                             schema=sample_definition_json_schema)
 
 
+@pytest.mark.skip_global_cleanup
+@pytest.mark.parametrize("guided_decoding_backend", GUIDED_DECODING_BACKENDS)
+def test_guided_enum_json_completion(sample_enum_json_schema, llm,
+                                     guided_decoding_backend: str):
+    sampling_params = SamplingParams(temperature=1.0,
+                                     max_tokens=1000,
+                                     guided_decoding=GuidedDecodingParams(
+                                         json=sample_enum_json_schema,
+                                         backend=guided_decoding_backend))
+    outputs = llm.generate(prompts=[
+        "Create a bug report JSON that fits this schema: "
+        f"{sample_enum_json_schema}. Make it for a high priority critical bug."
+    ] * 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_enum_json_schema)
+
+        # Additional assertions to verify enum values
+        assert output_json["status"] in ["active", "inactive", "pending"]
+        assert output_json["priority"] in ["low", "medium", "high", "critical"]
+        assert output_json["category"]["type"] in [
+            "bug", "feature", "improvement"
+        ]
+        assert output_json["category"]["severity"] in [1, 2, 3, 4, 5]
+        for flag in output_json["flags"]:
+            assert flag in ["urgent", "blocked", "needs_review", "approved"]
+
+
 @pytest.mark.skip_global_cleanup
 @pytest.mark.parametrize("guided_decoding_backend", GUIDED_DECODING_BACKENDS)
 def test_guided_choice_completion(sample_guided_choice, llm,

vllm/model_executor/guided_decoding/utils.py

@@ -14,6 +14,10 @@ def has_xgrammar_unsupported_json_features(schema: dict) -> bool:
         if "pattern" in obj:
             return True
 
+        # Check for enum restrictions
+        if "enum" in obj:
+            return True
+
         # Check for numeric ranges
         if obj.get("type") in ("integer", "number") and any(
                 key in obj for key in [