diff --git a/electron/main.mjs b/electron/main.mjs
index 083ef6f7..f4f84518 100644
--- a/electron/main.mjs
+++ b/electron/main.mjs
@@ -17,6 +17,19 @@ const DEV_URL = process.env.ELECTRON_START_URL ?? "http://127.0.0.1:5199";
 let SERVER_PORT = 8799;
 const APP_ICON = path.join(__dirname, "resources/app-icon.png");
 
+const gotSingleInstanceLock = app.requestSingleInstanceLock();
+if (!gotSingleInstanceLock) {
+  app.quit();
+} else {
+  app.on("second-instance", () => {
+    const win = BrowserWindow.getAllWindows()[0];
+    if (!win) return;
+    if (win.isMinimized()) win.restore();
+    win.show();
+    win.focus();
+  });
+}
+
 // GNOME groups the window with its installed desktop entry only when both
 // identities match. This must run before Electron becomes ready.
 if (process.platform === "linux") app.setDesktopName("com.openmausbot.app.desktop");
@@ -29,6 +42,7 @@ if (process.platform === "linux") app.setDesktopName("com.openmausbot.app.deskto
 // our API shape, not just a 200).
 let serverProc = null;
 let serverReady = true;
+let isQuitting = false;
 
 // The packaged app has no terminal: everything about the server child's life
 // goes to server.log in the OS log dir (~/Library/Logs/OpenMausBot on macOS,
@@ -79,7 +93,15 @@ async function startServerOn(port) {
       const res = await fetch(`http://127.0.0.1:${port}/api/health`);
       if (res.ok) {
         const body = await res.json().catch(() => null);
-        if (body?.app === "openmausbot" && body.pid === proc.pid && body.static) return proc;
+        // The child can exit while the response body is being parsed; an exit
+        // watcher registered after that point would never observe the event.
+        if (exited) return null;
+        if (body?.app === "openmausbot" && body.pid === proc.pid && body.static) {
+          proc.once("exit", () => {
+            if (!isQuitting) app.quit();
+          });
+          return proc;
+        }
         break; // someone else owns this port — try the next one
       }
     } catch {
@@ -285,7 +307,7 @@ ipcMain.handle("desktop:capabilities", async () =>
   }),
 );
 
-app.whenReady().then(async () => {
+if (gotSingleInstanceLock) app.whenReady().then(async () => {
   if (process.platform === "darwin") app.dock.setIcon(APP_ICON);
   // getDisplayMedia in the renderer → this handler → ScreenCaptureKit, all
   // inside the app's own processes — the one capture path macOS reliably
@@ -334,6 +356,7 @@ app.on("window-all-closed", () => {
 const CUA_STOP_TIMEOUT_MS = 2500;
 let cuaCleanedUp = false;
 app.on("before-quit", (e) => {
+  isQuitting = true;
   if (cuaCleanedUp) return;
   e.preventDefault();
   try {
diff --git a/electron/main.test.mjs b/electron/main.test.mjs
new file mode 100644
index 00000000..edba06ff
--- /dev/null
+++ b/electron/main.test.mjs
@@ -0,0 +1,231 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const electron = vi.hoisted(() => {
+  class Emitter {
+    #listeners = new Map();
+
+    on(event, listener) {
+      const listeners = this.#listeners.get(event) ?? [];
+      listeners.push(listener);
+      this.#listeners.set(event, listeners);
+      return this;
+    }
+
+    once(event, listener) {
+      const wrapped = (...args) => {
+        this.#listeners.set(
+          event,
+          (this.#listeners.get(event) ?? []).filter((candidate) => candidate !== wrapped),
+        );
+        listener(...args);
+      };
+      return this.on(event, wrapped);
+    }
+
+    emit(event, ...args) {
+      for (const listener of [...(this.#listeners.get(event) ?? [])]) listener(...args);
+      return true;
+    }
+
+    removeAllListeners() {
+      this.#listeners.clear();
+    }
+  }
+
+  const app = Object.assign(new Emitter(), {
+    isPackaged: false,
+    requestSingleInstanceLock: vi.fn(() => true),
+    quit: vi.fn(),
+    whenReady: vi.fn(() => Promise.resolve()),
+    getPath: vi.fn((name) => `/tmp/openmausbot-${name}`),
+    setDesktopName: vi.fn(),
+    dock: { setIcon: vi.fn() },
+  });
+
+  class MockBrowserWindow extends Emitter {
+    static windows = [];
+
+    constructor() {
+      super();
+      this.webContents = {
+        setWindowOpenHandler: vi.fn(),
+        once: vi.fn(),
+      };
+      this.isMinimized = vi.fn(() => false);
+      this.restore = vi.fn();
+      this.show = vi.fn();
+      this.focus = vi.fn();
+      this.loadURL = vi.fn();
+      this.close = vi.fn();
+      MockBrowserWindow.windows.push(this);
+    }
+
+    static getAllWindows() {
+      return MockBrowserWindow.windows;
+    }
+
+    static fromWebContents() {
+      return null;
+    }
+  }
+
+  const ipcMain = { handle: vi.fn() };
+  const utilityProcess = { fork: vi.fn() };
+  const session = { defaultSession: { setDisplayMediaRequestHandler: vi.fn() } };
+  const desktopCapturer = { getSources: vi.fn(async () => []) };
+  const shell = { openExternal: vi.fn() };
+  const clipboard = { writeText: vi.fn() };
+  const systemPreferences = {};
+
+  return {
+    app,
+    Emitter,
+    BrowserWindow: MockBrowserWindow,
+    clipboard,
+    desktopCapturer,
+    ipcMain,
+    session,
+    shell,
+    systemPreferences,
+    utilityProcess,
+  };
+});
+
+vi.mock("electron", () => electron);
+vi.mock("./cua.mjs", () => ({
+  registerCuaIpc: vi.fn(),
+  startCua: vi.fn(async () => ({ mode: "unavailable" })),
+  stopCua: vi.fn(async () => {}),
+}));
+vi.mock("./speech.mjs", () => ({ finishSpeech: vi.fn(), startSpeech: vi.fn(), stopSpeech: vi.fn() }));
+vi.mock("./terminal-launch.mjs", () => ({ openBlankTerminal: vi.fn() }));
+vi.mock("./updater.mjs", () => ({ registerUpdaterIpc: vi.fn(), startUpdater: vi.fn() }));
+vi.mock("./capabilities.cjs", () => ({ default: { desktopCapabilities: vi.fn() } }));
+
+const loadMain = async () => {
+  vi.resetModules();
+  return import("./main.mjs");
+};
+
+describe("Electron app lifecycle", () => {
+  beforeEach(() => {
+    electron.app.removeAllListeners();
+    electron.app.isPackaged = false;
+    electron.app.requestSingleInstanceLock.mockReset();
+    electron.app.requestSingleInstanceLock.mockReturnValue(true);
+    electron.app.quit.mockReset();
+    electron.app.whenReady.mockReset();
+    electron.app.whenReady.mockReturnValue(Promise.resolve());
+    electron.utilityProcess.fork.mockReset();
+    electron.BrowserWindow.windows = [];
+    process.resourcesPath = "/tmp/openmausbot-resources";
+  });
+
+  it("exits a second launch before readiness or server startup", async () => {
+    electron.app.requestSingleInstanceLock.mockReturnValue(false);
+
+    await loadMain();
+
+    expect(electron.app.quit).toHaveBeenCalledOnce();
+    expect(electron.app.whenReady).not.toHaveBeenCalled();
+    expect(electron.utilityProcess.fork).not.toHaveBeenCalled();
+  });
+
+  it("activates the existing window when a second instance starts", async () => {
+    await loadMain();
+    await new Promise((resolve) => setImmediate(resolve));
+
+    const win = electron.BrowserWindow.getAllWindows()[0];
+    win.isMinimized.mockReturnValue(true);
+    electron.app.emit("second-instance");
+
+    expect(win.restore).toHaveBeenCalledOnce();
+    expect(win.show).toHaveBeenCalledOnce();
+    expect(win.focus).toHaveBeenCalledOnce();
+  });
+
+  it("quits the host when the successfully started server exits", async () => {
+    electron.app.isPackaged = true;
+    globalThis.fetch = vi.fn(async () => ({
+      ok: true,
+      json: async () => ({ app: "openmausbot", pid: 123, static: true }),
+    }));
+    const proc = Object.assign(new electron.Emitter(), {
+      pid: 123,
+      stdout: new electron.Emitter(),
+      stderr: new electron.Emitter(),
+      kill: vi.fn(),
+    });
+    electron.utilityProcess.fork.mockReturnValue(proc);
+
+    await loadMain();
+    await vi.waitFor(() => expect(electron.utilityProcess.fork).toHaveBeenCalledOnce());
+    proc.emit("exit", 1);
+
+    expect(electron.app.quit).toHaveBeenCalledOnce();
+  });
+
+  it("retries when the server exits while its health body is being parsed", async () => {
+    electron.app.isPackaged = true;
+    let resolveFirstHealth;
+    const firstHealth = new Promise((resolve) => {
+      resolveFirstHealth = resolve;
+    });
+    globalThis.fetch = vi
+      .fn()
+      .mockResolvedValueOnce({ ok: true, json: () => firstHealth })
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => ({ app: "openmausbot", pid: 456, static: true }),
+      });
+    const first = Object.assign(new electron.Emitter(), {
+      pid: 123,
+      stdout: new electron.Emitter(),
+      stderr: new electron.Emitter(),
+      kill: vi.fn(),
+    });
+    const second = Object.assign(new electron.Emitter(), {
+      pid: 456,
+      stdout: new electron.Emitter(),
+      stderr: new electron.Emitter(),
+      kill: vi.fn(),
+    });
+    electron.utilityProcess.fork.mockReturnValueOnce(first).mockReturnValueOnce(second);
+
+    await loadMain();
+    await vi.waitFor(() => expect(globalThis.fetch).toHaveBeenCalledOnce());
+    first.emit("exit", 1);
+    resolveFirstHealth({ app: "openmausbot", pid: 123, static: true });
+
+    await vi.waitFor(() => expect(electron.utilityProcess.fork).toHaveBeenCalledTimes(2));
+    await vi.waitFor(() => expect(electron.BrowserWindow.getAllWindows()[0]?.loadURL).toHaveBeenCalledWith(
+      "http://127.0.0.1:18799",
+    ));
+  });
+
+  it("does not re-quit when the server exits during intentional shutdown", async () => {
+    electron.app.isPackaged = true;
+    globalThis.fetch = vi.fn(async () => ({
+      ok: true,
+      json: async () => ({ app: "openmausbot", pid: 123, static: true }),
+    }));
+    const proc = Object.assign(new electron.Emitter(), {
+      pid: 123,
+      stdout: new electron.Emitter(),
+      stderr: new electron.Emitter(),
+      kill: vi.fn(),
+    });
+    electron.utilityProcess.fork.mockReturnValue(proc);
+
+    await loadMain();
+    await vi.waitFor(() => expect(electron.utilityProcess.fork).toHaveBeenCalledOnce());
+    const beforeQuit = { preventDefault: vi.fn() };
+    electron.app.emit("before-quit", beforeQuit);
+    await new Promise((resolve) => setImmediate(resolve));
+    proc.emit("exit", 1);
+    await new Promise((resolve) => setImmediate(resolve));
+
+    expect(beforeQuit.preventDefault).toHaveBeenCalledOnce();
+    expect(electron.app.quit).toHaveBeenCalledOnce();
+  });
+});