Tracer Bullets

Build one thin vertical slice through all system layers before widening. Tracer bullets are production code — not prototypes, not spikes. They validate architecture through real integration, then become the foundation for remaining work.

AI agents are especially prone to building horizontal layers in isolation, discovering integration failures only after committing to an unproven architecture.

When to Use

• Solution is NOT a well-trodden path — no established docs, tutorials, or prior art in the codebase
• Feature is large AND has a natural decomposition: MVP core + features on top + nice-to-haves
• Multiple system layers that haven't been integrated before in this codebase
• Uncertainty about whether the proposed architecture will actually work

When NOT to Use

• Simple CRUD with well-documented patterns
• Single-layer changes (only touches API, only touches UI)
• Small scope where the whole feature IS the thin slice
• Technology and integration patterns are already proven in this codebase