I just tried it in ChatGPT "Auto" and it didn't work

> Yes — ((((()))))) is balanced.

> It has 6 opening ( and 6 closing ), and they’re properly nested.

Though it did work when using "Extensive Thinking". The model wrote a Python program to solve this.

> Almost balanced — ((((()))))) has 5 opening parentheses and 6 closing parentheses, so it has one extra ).

> A balanced version would be: ((((()))))

Testing a couple of different models without a harness such that no tool calls are possible would be interesting