Fascinating concept, you essentially frame the backend not as a static codebase, but as an adaptive organism that evolves based on real-time usage.

A few things that come to my mind if I were to build this:

The 'Agent-User' Paradox: To make this work, you'd need the initial agents (the ones responding and testing the goals) to be 'chaotic' enough to explore edge cases, but 'structured' enough to provide meaningful feedback to the 'Architect' agent.

The Schema Contract: How would you ensure that as the backend "writes itself," it doesn't break the contract with the frontend? You’d almost need a JIT Documentation layer that updates in lockstep.

Verification: I wonder if the server should run the 'JIT-ed' code in a sandbox first, using the telemetry to verify the goal was met before promoting the code to the main branch.

It’s a massive shift from Code as an Asset to Code as a Runtime Behavior. Have you thought about how you'd handle state/database migrations in a world where the backend is rewriting itself on the fly? It feels to me that you're almost building a lovable for backend services. I've seen a few OS projects like this (e.g. MotiaDev) But none has executed this perfectly yet.