"Closed loop" is a defined engineering pattern, not a metaphor. IBM formalized it in 2003 as MAPE-K: Monitor, Analyze, Plan, Execute, sharing Knowledge. Every system that actually closes the loop is an instance of this pattern.
In my latest post, I walk through what each MAPE-K layer needs to look like from first principles, with OpenAI's recent data agent as the case study. If you're designing toward Hu or Blomfield's RFS, start here before you start integrating tools.