Test driven developmet practitioners in Smalltalk used to (still do?) write the test before writing the implementation, run the test, and, when the "method not implemented" error shows up in the debugger, type the implementation and continue the execution. Common Lisp can do that too.

So, in practice, it might look like you hit any kind of runtime failure, and then the LLM writes some code to fix it, and the user's request completes successfully with no errors.