Claude can't click through REAPER, and it can't hear. It can drive REAPER through REAPER's own scripting API, and it can measure audio. So each piece is written as code that builds a REAPER session. REAPER renders it, a script measures the result, and you listen and give your verdict.
song file (code) → session file + job → REAPER builds and renders it → measure → you listen → revise