[V1] Set structured output backend to auto by default

The previous default was `xgrammar` and users had to opt-in to fallback
behavior. After more thought, `auto` seems like a better default as it
lets us do our best to satisfy all requests. Users can still pin vllm to
a single backend if desired.

Make `auto` work for V0 in case it gets specified there, as well.

Signed-off-by: Russell Bryant <rbryant@redhat.com>

Author: russellb

vllm/config.py

@@ -2888,7 +2888,7 @@ class DecodingConfig:
 
     # Which guided decoding algo to use.
     # 'outlines' / 'lm-format-enforcer' / 'xgrammar'
-    guided_decoding_backend: str = 'xgrammar'
+    guided_decoding_backend: str = "auto" if envs.VLLM_USE_V1 else "xgrammar"
 
     reasoning_backend: Optional[str] = None
 
@@ -2913,7 +2913,7 @@ def compute_hash(self) -> str:
 
     def __post_init__(self):
         v0_valid_guided_backends = [
-            'outlines', 'lm-format-enforcer', 'xgrammar'
+            'outlines', 'lm-format-enforcer', 'xgrammar', 'auto'
         ]
         v1_valid_guided_backends = ['xgrammar', 'guidance', 'auto']
 

vllm/engine/arg_utils.py

@@ -181,7 +181,7 @@ class EngineArgs:
     enable_chunked_prefill: Optional[bool] = None
     disable_chunked_mm_input: bool = False
 
-    guided_decoding_backend: str = 'xgrammar'
+    guided_decoding_backend: str = DecodingConfig.guided_decoding_backend
     logits_processor_pattern: Optional[str] = None
 
     speculative_config: Optional[Dict[str, Any]] = None
@@ -381,13 +381,13 @@ def add_cli_args(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
         parser.add_argument(
             '--guided-decoding-backend',
             type=str,
-            default='xgrammar',
+            default=DecodingConfig.guided_decoding_backend,
             help='Which engine will be used for guided decoding'
             ' (JSON schema / regex etc) by default. Currently support '
             'https://github.com/mlc-ai/xgrammar and '
             'https://github.com/guidance-ai/llguidance.'
             'Valid backend values are "xgrammar", "guidance", and "auto". '
-            'With "auto", we will make opinionated choices based on request'
+            'With "auto", we will make opinionated choices based on request '
             'contents and what the backend libraries currently support, so '
             'the behavior is subject to change in each release.')
         parser.add_argument(

vllm/model_executor/guided_decoding/__init__.py

@@ -33,6 +33,12 @@ def fallback_or_error(guided_params: GuidedDecodingParams, message: str,
         logger.warning("%s Falling back to use %s instead.", message, fallback)
         guided_params.backend = fallback
 
+    # `auto` was added for V1 to explicitly declare a mode that has fallbacks
+    # in place. If that is specified with V0, treat it as `xgrammar`, as we have
+    # fallbacks enabled for that and it is the V0 default.
+    if guided_params.backend == "auto":
+        guided_params.backend = "xgrammar"
+
     # lm-format-enforce doesn't support grammar, fallback to xgrammar
     if guided_params.backend_name == "lm-format-enforcer":
         if guided_params.grammar is not None: