diff --git a/api/core/workflow/nodes/http_request/http_executor.py b/api/core/workflow/nodes/http_request/http_executor.py
index cb636df2f3..17238da11a 100644
--- a/api/core/workflow/nodes/http_request/http_executor.py
+++ b/api/core/workflow/nodes/http_request/http_executor.py
@@ -378,6 +378,9 @@ class HttpExecutor:
                 target_value_type=ValueType.STRING
             )
 
+            if value is None:
+                raise ValueError(f'Variable {variable_selector.variable} not found')
+
             if escape_quotes:
                 value = value.replace('"', '\\"')
 
diff --git a/api/services/workflow/workflow_converter.py b/api/services/workflow/workflow_converter.py
index 0c5f453208..86c30bd33f 100644
--- a/api/services/workflow/workflow_converter.py
+++ b/api/services/workflow/workflow_converter.py
@@ -288,7 +288,7 @@ class WorkflowConverter:
                     'app_id': app_model.id,
                     'tool_variable': tool_variable,
                     'inputs': inputs,
-                    'query': '{{_query}}' if app_model.mode == AppMode.CHAT.value else ''
+                    'query': '{{#sys.query#}}' if app_model.mode == AppMode.CHAT.value else ''
                 }
             }
 
@@ -389,7 +389,7 @@ class WorkflowConverter:
                         }
                     }
                 }
-                if retrieve_config.retrieve_strategy == DatasetRetrieveConfigEntity.RetrieveStrategy.MULTIPLE
+                if retrieve_config.retrieve_strategy == DatasetRetrieveConfigEntity.RetrieveStrategy.SINGLE
                 else None,
                 "multiple_retrieval_config": {
                     "top_k": retrieve_config.top_k,