diff --git a/docs/module-layout.md b/docs/module-layout.md
new file mode 100644
index 0000000..5bcb1f7
--- /dev/null
+++ b/docs/module-layout.md
@@ -0,0 +1,14 @@
+# Module layout roadmap
+
+`src/index.ts` currently hosts routing, middleware, stores, and handlers in
+one file (issue #13). The intended split:
+
+| Module | Responsibility |
+|--------|----------------|
+| `src/routers/` | Express route mounts (`pairs`, `webhooks`, `admin`) |
+| `src/middleware/` | CORS, rate limit, security headers, pause guard |
+| `src/stores.ts` | In-memory registries (already extracted) |
+| `src/validation/` | Shared request validators |
+
+New endpoints should prefer small router modules over growing `index.ts`.
+See `docs/middleware-order.md` and `docs/architecture.md` for current flow.