fix: misleading ask_user error when questions are not sent as a list (#32727)

When a model sent the ask_user questions as anything other than a list, for example a JSON-encoded string, the tool answered "ask_user requires 1-3 questions." The model had sent the right number of questions, so it did not understand the error and often repeated the same call in a loop. It now gets a separate error saying the questions must be an array of question objects, and the count error only appears when the count is actually wrong.

Fixes #32662

backend/open_webui/tools/builtin.py

@@ -539,7 +539,9 @@ async def ask_user(
     :return: JSON with status and answers keyed by question id
     """
     try:
-        if not isinstance(questions, list) or not 1 <= len(questions) <= 3:
+        if not isinstance(questions, list):
+            raise ValueError('ask_user questions must be an array of question objects.')
+        if not 1 <= len(questions) <= 3:
             raise ValueError('ask_user requires 1-3 questions.')
 
         normalized_questions = []

backend/open_webui/utils/ask_user.py

@@ -24,7 +24,9 @@ def get_ask_user_tool_calls(tool_calls: list[dict]) -> tuple[list[dict], str | N
 
 def normalize_ask_user_request(arguments: dict) -> dict:
     questions = arguments.get('questions')
-    if not isinstance(questions, list) or not 1 <= len(questions) <= 3:
+    if not isinstance(questions, list):
+        raise ValueError('ask_user questions must be an array of question objects.')
+    if not 1 <= len(questions) <= 3:
         raise ValueError('ask_user requires 1-3 questions.')
 
     normalized_questions = []