But if you observe that the agent day after day do handle user input safely; and also routinely run an agent that scans for security vulnerabilities and observe

But if you observe that the agent day after day do handle user input safely; and also routinely run an agent that scans for security vulnerabilities and observe it finding cases where input is not handled safely in existing code, you may conclude that the chance of an issue is at the same level at, or probably lower than, if a human wrote it and a human reviewed it. ("Escaping" user input is not good practice though, use parameters, assuming you are talking about SQL.)