fix: properly handle custom guardrails parameters (#18978)

litellm/proxy/guardrails/guardrail_registry.py

@@ -526,10 +526,24 @@ class InMemoryGuardrailHandler:
             )
 
         default_on = litellm_params.default_on
+
+        # Extract additional params from litellm_params to pass to custom guardrail
+        # This matches the behavior of other guardrail initializers (e.g., initialize_lakera)
+        # and aligns with the documented behavior for custom guardrails
+        if hasattr(litellm_params, "model_dump"):
+            extra_params = litellm_params.model_dump(exclude_none=True)
+        else:
+            extra_params = dict(litellm_params) if litellm_params else {}
+
+        # Remove params that are handled explicitly or are internal
+        for key in ["guardrail", "mode", "default_on"]:
+            extra_params.pop(key, None)
+
         _guardrail_callback = _guardrail_class(
             guardrail_name=guardrail["guardrail_name"],
             event_hook=mode,
             default_on=default_on,
+            **extra_params,
         )
         litellm.logging_callback_manager.add_litellm_callback(_guardrail_callback)  # type: ignore