diff --git a/web/src/layout.ts b/web/src/layout.ts
new file mode 100644
index 0000000..0e632e0
--- /dev/null
+++ b/web/src/layout.ts
@@ -0,0 +1,57 @@
+// Multi-view layout model. The layout tree is the single source of truth for
+// what's on screen: a set of tabs, each holding a binary split tree whose leaves
+// are sessions. Desktop renders the tree as nested splits; mobile projects its
+// leaves to tabs. (Splits/tabs UI land in later issues — see epic #37. This
+// issue introduces the model and renders a single-leaf tree at parity.)
+
+import type { Uuid } from "./types.js";
+
+export type Dir = "row" | "col";
+
+export type Node =
+  | { kind: "leaf"; id: string; sessionId: Uuid }
+  | { kind: "split"; dir: Dir; a: Node; b: Node; ratio: number };
+
+export type Leaf = Extract<Node, { kind: "leaf" }>;
+
+export interface Tab {
+  id: string;
+  title: string;
+  /** The split tree, or null for an empty tab. */
+  root: Node | null;
+  /** Which leaf has keyboard focus / drives the sidebar highlight. */
+  focusedLeafId: string | null;
+}
+
+export interface Layout {
+  tabs: Tab[];
+  activeTabId: string;
+}
+
+let seq = 0;
+/** Monotonic id for tabs and leaves (not persisted across reloads — restore
+ *  assigns fresh ids). */
+export function freshId(prefix = "n"): string {
+  return `${prefix}${++seq}`;
+}
+
+/** Every leaf under `node`, left-to-right. */
+export function leaves(node: Node | null): Leaf[] {
+  if (!node) return [];
+  if (node.kind === "leaf") return [node];
+  return [...leaves(node.a), ...leaves(node.b)];
+}
+
+export function findLeaf(node: Node | null, leafId: string): Leaf | null {
+  return leaves(node).find((l) => l.id === leafId) ?? null;
+}
+
+export function activeTab(layout: Layout): Tab {
+  return layout.tabs.find((t) => t.id === layout.activeTabId) ?? layout.tabs[0];
+}
+
+/** A layout with one empty tab. */
+export function emptyLayout(): Layout {
+  const tab: Tab = { id: freshId("t"), title: "Tab 1", root: null, focusedLeafId: null };
+  return { tabs: [tab], activeTabId: tab.id };
+}