diff --git a/src/lib/store.test.ts b/src/lib/store.test.ts
new file mode 100644
index 0000000..286880b
--- /dev/null
+++ b/src/lib/store.test.ts
@@ -0,0 +1,60 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { useOverlayStore } from "./store";
+
+const initial = useOverlayStore.getState();
+
+describe("useOverlayStore", () => {
+  beforeEach(() => useOverlayStore.setState(initial, true));
+
+  it("starts with sensible defaults", () => {
+    const s = useOverlayStore.getState();
+    expect(s.url).toBe("");
+    expect(s.opacity).toBe(1.0);
+    expect(s.alwaysOnTop).toBe(true);
+    expect(s.clickThrough).toBe(false);
+    expect(s.isOpen).toBe(false);
+    expect(s.urlError).toBe(null);
+    expect(s.hotkeyError).toBe(null);
+  });
+
+  it("setUrl validates and stores both url and urlError", () => {
+    useOverlayStore.getState().setUrl("twitch.tv");
+    expect(useOverlayStore.getState().url).toBe("twitch.tv");
+    expect(useOverlayStore.getState().urlError).toBe(null);
+
+    useOverlayStore.getState().setUrl("javascript:alert(1)");
+    expect(useOverlayStore.getState().urlError).toMatch(/http and https/);
+  });
+
+  it("setOpacity clamps to 0.1..1.0", () => {
+    useOverlayStore.getState().setOpacity(0.05);
+    expect(useOverlayStore.getState().opacity).toBe(0.1);
+    useOverlayStore.getState().setOpacity(2);
+    expect(useOverlayStore.getState().opacity).toBe(1.0);
+    useOverlayStore.getState().setOpacity(0.5);
+    expect(useOverlayStore.getState().opacity).toBe(0.5);
+  });
+
+  it("setAlwaysOnTop and setClickThrough mutate flags", () => {
+    useOverlayStore.getState().setAlwaysOnTop(false);
+    expect(useOverlayStore.getState().alwaysOnTop).toBe(false);
+    useOverlayStore.getState().setClickThrough(true);
+    expect(useOverlayStore.getState().clickThrough).toBe(true);
+  });
+
+  it("hydrate replaces persisted fields and clears errors", () => {
+    useOverlayStore.getState().setUrl("javascript:alert(1)");
+    useOverlayStore.getState().hydrate({
+      url: "https://example.com",
+      opacity: 0.6,
+      alwaysOnTop: false,
+      clickThrough: true,
+    });
+    const s = useOverlayStore.getState();
+    expect(s.url).toBe("https://example.com");
+    expect(s.opacity).toBe(0.6);
+    expect(s.alwaysOnTop).toBe(false);
+    expect(s.clickThrough).toBe(true);
+    expect(s.urlError).toBe(null);
+  });
+});
diff --git a/src/lib/store.ts b/src/lib/store.ts
new file mode 100644
index 0000000..39a5ade
--- /dev/null
+++ b/src/lib/store.ts
@@ -0,0 +1,46 @@
+import { create } from "zustand";
+import type { OverlayActions, OverlayPersistedState, OverlayState } from "../types/overlay";
+import { validateUrl } from "./url";
+
+const clamp = (v: number, min: number, max: number): number =>
+  Math.min(max, Math.max(min, v));
+
+export const useOverlayStore = create<OverlayState & OverlayActions>()((set) => ({
+  url: "",
+  opacity: 1.0,
+  alwaysOnTop: true,
+  clickThrough: false,
+  isOpen: false,
+  urlError: null,
+  hotkeyError: null,
+
+  setUrl: (raw) => {
+    const result = validateUrl(raw);
+    set({
+      url: raw,
+      urlError: result.ok ? null : raw.trim() === "" ? null : result.error,
+    });
+  },
+
+  setOpacity: (v) => set({ opacity: clamp(v, 0.1, 1.0) }),
+
+  setAlwaysOnTop: (v) => set({ alwaysOnTop: v }),
+
+  setClickThrough: (v) => set({ clickThrough: v }),
+
+  _setClickThroughFromBackend: (v) =>
+    set((s) => (s.clickThrough === v ? s : { clickThrough: v })),
+
+  setHotkeyError: (msg) => set({ hotkeyError: msg }),
+
+  setIsOpen: (v) => set({ isOpen: v }),
+
+  hydrate: (p: OverlayPersistedState) =>
+    set({
+      url: p.url,
+      opacity: clamp(p.opacity, 0.1, 1.0),
+      alwaysOnTop: p.alwaysOnTop,
+      clickThrough: p.clickThrough,
+      urlError: null,
+    }),
+}));