Simplify (and fix) passing of guided decoding backend options (#17008)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config.py

@@ -17,12 +17,14 @@ from dataclasses import (MISSING, dataclass, field, fields, is_dataclass,
 from importlib.util import find_spec
 from pathlib import Path
 from typing import (TYPE_CHECKING, Any, Callable, ClassVar, Final, Literal,
-                    Optional, Protocol, TypeVar, Union, get_args, get_origin)
+                    Optional, Protocol, TypeVar, Union, cast, get_args,
+                    get_origin)
 
 import torch
 from pydantic import BaseModel, Field, PrivateAttr
 from torch.distributed import ProcessGroup, ReduceOp
 from transformers import PretrainedConfig
+from typing_extensions import deprecated
 
 import vllm.envs as envs
 from vllm.compilation.inductor_pass import CallableInductorPass, InductorPass
@@ -32,7 +34,6 @@ from vllm.model_executor.layers.quantization import (QUANTIZATION_METHODS,
                                                      get_quantization_config)
 from vllm.model_executor.models import ModelRegistry
 from vllm.platforms import CpuArchEnum, current_platform
-from vllm.sampling_params import GuidedDecodingParams
 from vllm.tracing import is_otel_available, otel_import_error_traceback
 from vllm.transformers_utils.config import (
     ConfigFormat, get_config, get_hf_image_processor_config,
@@ -344,7 +345,7 @@ class ModelConfig:
     def __init__(
         self,
         model: str,
-        task: Union[TaskOption, Literal["draft"]],
+        task: Literal[TaskOption, Literal["draft"]],
         tokenizer: str,
         tokenizer_mode: str,
         trust_remote_code: bool,
@@ -701,7 +702,7 @@ class ModelConfig:
 
     def _resolve_task(
         self,
-        task_option: Union[TaskOption, Literal["draft"]],
+        task_option: Literal[TaskOption, Literal["draft"]],
     ) -> tuple[set[_ResolvedTask], _ResolvedTask]:
         if task_option == "draft":
             return {"draft"}, "draft"
@@ -3185,13 +3186,36 @@ GuidedDecodingBackend = Literal[GuidedDecodingBackendV0,
 class DecodingConfig:
     """Dataclass which contains the decoding strategy of the engine."""
 
-    guided_decoding_backend: GuidedDecodingBackend = \
-        "auto" if envs.VLLM_USE_V1 else "xgrammar"
+    @property
+    @deprecated(
+        "`guided_decoding_backend` is deprecated and has been renamed to "
+        "`backend`. This will be removed in v0.10.0. Please use the "
+        "`backend` argument instead.")
+    def guided_decoding_backend(self) -> GuidedDecodingBackend:
+        return self.backend
+
+    @guided_decoding_backend.setter
+    def guided_decoding_backend(self, value: GuidedDecodingBackend):
+        self.backend = value
+
+    backend: GuidedDecodingBackend = "auto" if envs.VLLM_USE_V1 else "xgrammar"
     """Which engine will be used for guided decoding (JSON schema / regex etc)
     by default. 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."""
 
+    disable_fallback: bool = False
+    """If `True`, vLLM will not fallback to a different backend on error."""
+
+    disable_any_whitespace: bool = False
+    """If `True`, the model will not generate any whitespace during guided
+    decoding. This is only supported for xgrammar and guidance backends."""
+
+    disable_additional_properties: bool = False
+    """If `True`, the `guidance` backend will not use `additionalProperties`
+    in the JSON schema. This is only supported for the `guidance` backend and
+    is used to better align its behaviour with `outlines` and `xgrammar`."""
+
     reasoning_backend: Optional[str] = None
     """Select the reasoning parser depending on the model that you're using.
     This is used to parse the reasoning content into OpenAI API format.
@@ -3217,15 +3241,41 @@ class DecodingConfig:
         return hash_str
 
     def __post_init__(self):
-        backend = GuidedDecodingParams(
-            backend=self.guided_decoding_backend).backend_name
+        if ":" in self.backend:
+            self._extract_backend_options()
+
         if envs.VLLM_USE_V1:
             valid_guided_backends = get_args(GuidedDecodingBackendV1)
         else:
             valid_guided_backends = get_args(GuidedDecodingBackendV0)
-        if backend not in valid_guided_backends:
-            raise ValueError(f"Invalid guided_decoding_backend '{backend}',"
+        if self.backend not in valid_guided_backends:
+            raise ValueError(f"Invalid backend '{self.backend}',"
                              f" must be one of {valid_guided_backends}")
+        if (self.disable_any_whitespace
+                and self.backend not in ("xgrammar", "guidance")):
+            raise ValueError("disable_any_whitespace is only supported for "
+                             "xgrammar and guidance backends.")
+        if (self.disable_additional_properties and self.backend != "guidance"):
+            raise ValueError("disable_additional_properties is only supported "
+                             "for the guidance backend.")
+
+    @deprecated(
+        "Passing guided decoding backend options inside backend in the format "
+        "'backend:...' is deprecated. This will be removed in v0.10.0. Please "
+        "use the dedicated arguments '--disable-fallback', "
+        "'--disable-any-whitespace' and '--disable-additional-properties' "
+        "instead.")
+    def _extract_backend_options(self):
+        """Extract backend options from the backend string."""
+        backend, options = self.backend.split(":")
+        self.backend = cast(GuidedDecodingBackend, backend)
+        options_set = set(options.strip().split(","))
+        if "no-fallback" in options_set:
+            self.disable_fallback = True
+        if "disable-any-whitespace" in options_set:
+            self.disable_any_whitespace = True
+        if "no-additional-properties" in options_set:
+            self.disable_additional_properties = True
 
 
 @dataclass