I find that it helps a little to maintain an iron grip on the design. Take the time for really detailed change planning. Be pedantic and push back against every little thing that seems wrong or even a little odd in any plan document the LLM spews out. Likewise, watch for the for the moment when the LLM eventually stumbles and starts to make excuses for deviations from the plan. Interrupt immediately and force it back onto the plan (or your vision), potentially with pointers to resolve immediate problems. Completely unattended agents just don't work for important code.