[Bugfix] Guided decoding falls back to outlines when fails to import xgrammar (#12976)

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

vllm/model_executor/guided_decoding/__init__.py

@@ -40,6 +40,8 @@ def maybe_backend_fallback(
             guided_params.backend = "outlines"
 
     if guided_params.backend == "xgrammar":
+        from vllm.model_executor.guided_decoding.xgrammar_decoding import (
+            xgr_installed)
         # xgrammar only has x86 wheels for linux, fallback to outlines
         from vllm.platforms import current_platform
         if current_platform.get_cpu_architecture() is not CpuArchEnum.X86:
@@ -77,6 +79,13 @@ def maybe_backend_fallback(
                     "Falling back to use outlines instead.")
                 guided_params.backend = "outlines"
 
+        # If the xgrammar module cannot be imported successfully,
+        # we should still allow users to use guided decoding with a fallback.
+        elif not xgr_installed:
+            logger.warning("xgrammar module cannot be imported successfully. "
+                           "Falling back to use outlines instead.")
+            guided_params.backend = "outlines"
+
     if (guided_params.backend == "outlines"
             and guided_params.json_object is not None):
         # outlines doesn't support json_object, fallback to xgrammar

vllm/model_executor/guided_decoding/xgrammar_decoding.py

@@ -14,7 +14,9 @@ from transformers import PreTrainedTokenizerFast
 try:
     import xgrammar as xgr
     from xgrammar.base import _core as xgr_core
+    xgr_installed = True
 except ImportError:
+    xgr_installed = False
     pass
 
 from vllm.model_executor.guided_decoding.utils import (convert_lark_to_gbnf,