Reduce validation to a warning (#28749)

Signed-off-by: Alec Solder <alecs@fb.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Alec Solder <alecs@fb.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/structured_outputs.py

@@ -65,22 +65,6 @@ class StructuredOutputsConfig:
 
     @model_validator(mode="after")
     def _validate_structured_output_config(self) -> Self:
-        # Import here to avoid circular import
-        from vllm.reasoning.abs_reasoning_parsers import ReasoningParserManager
-
-        if self.reasoning_parser_plugin and len(self.reasoning_parser_plugin) > 3:
-            ReasoningParserManager.import_reasoning_parser(self.reasoning_parser_plugin)
-
-        valid_reasoning_parsers = ReasoningParserManager.list_registered()
-        if (
-            self.reasoning_parser != ""
-            and self.reasoning_parser not in valid_reasoning_parsers
-        ):
-            raise ValueError(
-                f"invalid reasoning parser: {self.reasoning_parser} "
-                f"(chose from {{ {','.join(valid_reasoning_parsers)} }})"
-            )
-
         if self.disable_any_whitespace and self.backend not in ("xgrammar", "guidance"):
             raise ValueError(
                 "disable_any_whitespace is only supported for "

vllm/reasoning/abs_reasoning_parsers.py

@@ -160,7 +160,10 @@ class ReasoningParserManager:
         if name in cls.lazy_parsers:
             return cls._load_lazy_parser(name)
 
-        raise KeyError(f"Reasoning parser '{name}' not found.")
+        registered = ", ".join(cls.list_registered())
+        raise KeyError(
+            f"Reasoning parser '{name}' not found. Available parsers: {registered}"
+        )
 
     @classmethod
     def list_registered(cls) -> list[str]: