[Bugfix] Fallback to outlines for complex json schemas (#10899)

Signed-off-by: mgoin <michael@neuralmagic.com>

tests/entrypoints/conftest.py

@@ -69,6 +69,37 @@ def sample_json_schema():
     }
 
 
+@pytest.fixture
+def sample_complex_json_schema():
+    return {
+        "type": "object",
+        "properties": {
+            "score": {
+                "type": "integer",
+                "minimum": 0,
+                "maximum": 100  # Numeric range
+            },
+            "grade": {
+                "type": "string",
+                "pattern": "^[A-D]$"  # Regex pattern
+            },
+            "email": {
+                "type": "string",
+                "pattern": "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$"
+            },
+            "tags": {
+                "type": "array",
+                "items": {
+                    "type": "string",
+                    "pattern":
+                    "^[a-z]{1,10}$"  # Combining length and pattern restrictions
+                }
+            }
+        },
+        "required": ["score", "grade", "email", "tags"]
+    }
+
+
 @pytest.fixture
 def sample_guided_choice():
     return [

tests/entrypoints/llm/test_guided_generate.py

@@ -76,6 +76,34 @@ def test_guided_json_completion(sample_json_schema, llm):
         jsonschema.validate(instance=output_json, schema=sample_json_schema)
 
 
+@pytest.mark.skip_global_cleanup
+def test_guided_complex_json_completion(sample_complex_json_schema, llm):
+    sampling_params = SamplingParams(
+        temperature=1.0,
+        max_tokens=1000,
+        guided_decoding=GuidedDecodingParams(json=sample_complex_json_schema))
+    outputs = llm.generate(prompts=[
+        f"Give an example JSON for an assignment grade "
+        f"that fits this schema: {sample_complex_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_complex_json_schema)
+
+
 @pytest.mark.skip_global_cleanup
 def test_guided_choice_completion(sample_guided_choice, llm):
     sampling_params = SamplingParams(

vllm/model_executor/guided_decoding/__init__.py

@@ -15,6 +15,40 @@ if TYPE_CHECKING:
 logger = init_logger(__name__)
 
 
+def has_xgrammar_unsupported_json_features(schema: dict) -> bool:
+    """Check if JSON schema contains features unsupported by xgrammar."""
+
+    def check_object(obj: dict) -> bool:
+        if not isinstance(obj, dict):
+            return False
+
+        # Check for pattern restrictions
+        if "pattern" in obj:
+            return True
+
+        # Check for numeric ranges
+        if obj.get("type") in ("integer", "number") and any(
+                key in obj for key in [
+                    "minimum", "maximum", "exclusiveMinimum",
+                    "exclusiveMaximum", "multipleOf"
+                ]):
+            return True
+
+        # Recursively check all nested objects and arrays
+        for value in obj.values():
+            if isinstance(value, dict):
+                if check_object(value):
+                    return True
+            elif isinstance(value, list):
+                for item in value:
+                    if isinstance(item, dict) and check_object(item):
+                        return True
+
+        return False
+
+    return check_object(schema)
+
+
 def maybe_backend_fallback(
         guided_params: GuidedDecodingParams) -> GuidedDecodingParams:
     # lm-format-enforce doesn't support grammar, fallback to xgrammar
@@ -47,6 +81,15 @@ def maybe_backend_fallback(
                            "Falling back to use outlines instead.")
             guided_params.backend = "outlines"
 
+        # xgrammar doesn't support some JSON schema features
+        elif (guided_params.json is not None
+              and has_xgrammar_unsupported_json_features(guided_params.json)):
+            logger.warning(
+                "xgrammar does not support advanced JSON schema features like "
+                "patterns or numeric ranges. "
+                "Falling back to use outlines instead.")
+            guided_params.backend = "outlines"
+
     return guided_params