Architecture Drift in AI-Generated Codebases — How Clean Layers Dissolve One Prompt at a Time

"The architecture changed. Nobody decided it should." Architecture drift is the #1 root cause of structural failure in AI-generated codebases. It happens silently, one prompt at a time, and by the time it's visible, the cost of fixing it has compounded significantly. The Mechanism Each AI session optimizes for the immediate task without maintaining awareness of the broader architecture. BOUNDARY EROSION TIMELINE Month 1: API --> Business --> Data (clean layers, clear direction) Month 4: API <-> Business <-> Data (boundaries crossed, bidirectional) Month 8: API <-> Business <-> Data \ | X | / \---> UI <---/ (boundaries dissolved, spaghetti) Session 1: "Create a user service" — clean module with clear boundaries. Session 15: "Fix the checkout bug" — fix goes in the route handler because that's where the symptom is. Session 30: "Add email on purchase" — notification logic added inline in the payment handler. Session 50: Business logic lives in 5 layers.