AI app stack · Layer hierarchy

Layer stack · Diagram Design

# AI app stack · Where the work actually happens

Five layers between silicon and the user. Most teams over-invest in the model and under-invest in the harness — which is the layer that decides whether the app feels magical or flaky.

THE HEADLINE

### Coral marks the layer that pays rent

Swapping models is a knob. Rewriting the harness is a product decision. The coral band says: this is the layer where you out-execute the competition, not the one where you chase leaderboards.

### Reading the stack

- Abstraction rises up the left column
- L-index on the left, note on the right
- Hairlines between layers, no shadows
- Fill shade shifts from paper-2 to white

### Why only five

Six-plus layers become a legend, not a diagram. Five holds the whole thing on one screen — every band readable without squinting, every note a scan away.