diff --git a/CLAUDE.md b/CLAUDE.md
index 5cd2e44..a0436d5 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -454,6 +454,7 @@ tests/
   language-lookups.test.ts          # saved-phrase lookup, bundle suggestion, last-used bundles per language
   language-analytics.test.ts        # learning_language and target_language super properties
   tab-broadcast.test.ts             # tab broadcast skips tabs without our content scripts quietly
+  install-tracking.test.ts          # onInstalled: extension_installed / extension_updated + welcome page (E-02)
   e2e/
     extension-fixture.ts            # chromium.launchPersistentContext + extension load
     server.mjs                      # static fixtures HTTP server
diff --git a/src/background.ts b/src/background.ts
index 20a3a2f..ba80a19 100644
--- a/src/background.ts
+++ b/src/background.ts
@@ -9,6 +9,7 @@ import {
   MESSAGE_TYPE,
 } from "./common/types/messaging";
 import { ensureStoredTargetLanguage } from "./common/helper/default-language";
+import { handleInstalled } from "./common/helper/install-tracking";
 
 export {};
 
@@ -20,6 +21,14 @@ chrome.runtime.onInstalled.addListener(() => {
   );
 });
 
+// Track installs and updates on the install's analytics id, and open the
+// welcome page on a first install (E-02, see install-tracking.ts).
+chrome.runtime.onInstalled.addListener((details) => {
+  handleInstalled(details).catch((e) =>
+    log("background: install tracking failed", e)
+  );
+});
+
 function getOAuthToken() {
   return new Promise((resolve, reject) => {
     chrome.identity.getAuthToken({ interactive: true }, function (token) {
diff --git a/src/common/helper/install-id.ts b/src/common/helper/install-id.ts
new file mode 100644
index 0000000..cc14d3c
--- /dev/null
+++ b/src/common/helper/install-id.ts
@@ -0,0 +1,57 @@
+/**
+ * The install's analytics id (A3 in the analytics work list): one random id
+ * per install, kept in chrome.storage.local under `analyticsAnonymousId`.
+ *
+ * Mixpanel uses it as the anonymous `distinct_id` / `$device_id`
+ * (plugins/mixpanel.ts), and server calls that fire an analytics event send it
+ * as `deviceId` (E-01) so server events land on the same person.
+ *
+ * This module has no mixpanel import, so it is safe in every context: content
+ * scripts, the popup and the background service worker.
+ */
+
+export const ANON_ID_KEY = "analyticsAnonymousId";
+
+/** crypto.randomUUID is missing on plain-http pages; getRandomValues is not. */
+export function newAnonymousId(): string {
+  const b = crypto.getRandomValues(new Uint8Array(16));
+  b[6] = (b[6] & 0x0f) | 0x40;
+  b[8] = (b[8] & 0x3f) | 0x80;
+  const h = Array.from(b, (x) => x.toString(16).padStart(2, "0")).join("");
+  return `${h.slice(0, 8)}-${h.slice(8, 12)}-${h.slice(12, 16)}-${h.slice(16, 20)}-${h.slice(20)}`;
+}
+
+// One read-or-create at a time per context, so two callers on the first page
+// ever do not mint two different ids.
+let pending: Promise<string | undefined> | null = null;
+
+/**
+ * Read the stored install id, creating and storing it when there is none yet.
+ * Not cached after it settles: logout rotates the id (resetAnalyticsIdentity),
+ * and the next call must see the new one.
+ *
+ * Resolves `undefined` only when there is no extension context (e.g. the
+ * content script was orphaned by an update); callers then send no id.
+ */
+export function getInstallId(): Promise<string | undefined> {
+  if (pending) return pending;
+  pending = new Promise<string | undefined>((resolve) => {
+    try {
+      chrome.storage.local.get(ANON_ID_KEY, (data) => {
+        let id = data?.[ANON_ID_KEY] as string | undefined;
+        if (id) return resolve(id);
+        id = newAnonymousId();
+        try {
+          chrome.storage.local.set({ [ANON_ID_KEY]: id }, () => resolve(id));
+        } catch {
+          resolve(id);
+        }
+      });
+    } catch {
+      resolve(undefined);
+    }
+  }).finally(() => {
+    pending = null;
+  });
+  return pending;
+}
diff --git a/src/common/helper/install-tracking.ts b/src/common/helper/install-tracking.ts
new file mode 100644
index 0000000..db15d88
--- /dev/null
+++ b/src/common/helper/install-tracking.ts
@@ -0,0 +1,125 @@
+/**
+ * Install and update tracking for the background service worker (E-02).
+ *
+ * mixpanel-browser is not loaded here: it expects `window` and `document`,
+ * which a MV3 service worker does not have. So the two events go straight to
+ * Mixpanel's /track endpoint with fetch, in the same shape mixpanel-browser
+ * sends (base64 JSON in a `data` form field), through the same
+ * MIXPANEL_API_HOST proxy, on the install's analytics id. The content scripts
+ * use that same id as their anonymous `distinct_id` (plugins/mixpanel.ts), so
+ * these events join the rest of the install's events.
+ */
+import { getInstallId } from "./install-id";
+import { log } from "./log";
+
+/** The welcome page opened on first install (landing D-01 reads install_id). */
+export const WELCOME_URL = "https://subturtle.app/";
+
+export function welcomeUrl(installId: string): string {
+  const params = new URLSearchParams({
+    install_id: installId,
+    utm_source: "extension",
+    utm_medium: "install",
+  });
+  return `${WELCOME_URL}?${params.toString()}`;
+}
+
+const DEFAULT_API_HOST = "https://api-js.mixpanel.com";
+
+function insertId(): string {
+  const b = crypto.getRandomValues(new Uint8Array(12));
+  return Array.from(b, (x) => x.toString(16).padStart(2, "0")).join("");
+}
+
+/** UTF-8 safe base64, the encoding mixpanel-browser 2.45 uses for `data`. */
+function base64Json(value: unknown): string {
+  const bytes = new TextEncoder().encode(JSON.stringify(value));
+  let bin = "";
+  bytes.forEach((b) => (bin += String.fromCharCode(b)));
+  return btoa(bin);
+}
+
+/**
+ * Send one event to Mixpanel from a context without mixpanel-browser.
+ * Never throws: analytics must not break install handling.
+ */
+export async function trackFromBackground(
+  event: string,
+  installId: string,
+  properties: Record<string, unknown> = {}
+): Promise<void> {
+  const token = process.env.MIXPANEL_PROJECT_TOKEN;
+  if (!token) return;
+  const host = (process.env.MIXPANEL_API_HOST || DEFAULT_API_HOST).replace(
+    /\/+$/,
+    ""
+  );
+  const manifest = chrome.runtime.getManifest();
+  const payload = {
+    event,
+    properties: {
+      token,
+      // Same identity the content scripts register for an anonymous install.
+      distinct_id: installId,
+      $device_id: installId,
+      time: Date.now() / 1000,
+      $insert_id: insertId(),
+      app: "chrome-extension",
+      version: manifest.version_name || manifest.version,
+      ...properties,
+    },
+  };
+  try {
+    await fetch(`${host}/track/?ip=1`, {
+      method: "POST",
+      headers: { "Content-Type": "application/x-www-form-urlencoded" },
+      body: `data=${encodeURIComponent(base64Json(payload))}`,
+    });
+  } catch (e) {
+    log("background: analytics request failed", e);
+  }
+}
+
+/**
+ * Web Store installs carry `update_url` in the manifest; an unpacked build
+ * (local dev, the CI e2e run, a dev zip loaded by hand) does not.
+ */
+function isStoreInstall(): boolean {
+  return "update_url" in chrome.runtime.getManifest();
+}
+
+/**
+ * chrome.runtime.onInstalled handler: `extension_installed` on a new install
+ * (and open the welcome page), `extension_updated` with `previous_version` on
+ * an update. Other reasons (a Chrome update) are ignored.
+ */
+export async function handleInstalled(
+  details: chrome.runtime.InstalledDetails
+): Promise<void> {
+  const { reason, previousVersion } = details;
+  if (reason !== "install" && reason !== "update") return;
+
+  const installId = await getInstallId();
+  if (!installId) return;
+
+  if (reason === "install") {
+    // Open the page first: it must not wait on the analytics request.
+    // Unpacked builds skip it, so dev reloads and CI runs do not open
+    // subturtle.app or count fake installs on the landing page.
+    if (isStoreInstall()) {
+      try {
+        await chrome.tabs.create({ url: welcomeUrl(installId) });
+      } catch (e) {
+        log("background: failed to open the welcome page", e);
+      }
+    }
+    await trackFromBackground("extension_installed", installId, {
+      reason: "install",
+    });
+  } else {
+    await trackFromBackground("extension_updated", installId, {
+      reason: "update",
+      previous_version: previousVersion || null,
+    });
+  }
+}
diff --git a/tests/install-tracking.test.ts b/tests/install-tracking.test.ts
new file mode 100644
index 0000000..f49445d
--- /dev/null
+++ b/tests/install-tracking.test.ts
@@ -0,0 +1,132 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+
+import {
+  handleInstalled,
+  welcomeUrl,
+} from "../src/common/helper/install-tracking";
+import { ANON_ID_KEY } from "../src/common/helper/install-id";
+
+// E-02: chrome.runtime.onInstalled tracks extension_installed /
+// extension_updated on the install id, and opens the welcome page on install.
+
+const UUID_V4 =
+  /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/;
+
+function memoryChromeStorage() {
+  const mem: Record<string, any> = {};
+  vi.mocked(chrome.storage.local.get).mockImplementation(((
+    key: string,
+    cb: (d: any) => void
+  ) => cb(key in mem ? { [key]: mem[key] } : {})) as any);
+  vi.mocked(chrome.storage.local.set).mockImplementation(((
+    obj: Record<string, any>,
+    cb?: () => void
+  ) => {
+    Object.assign(mem, obj);
+    cb?.();
+  }) as any);
+  return mem;
+}
+
+/** Decode the events posted to the mixpanel /track endpoint. */
+function sentEvents(fetchMock: ReturnType<typeof vi.fn>) {
+  return fetchMock.mock.calls.map(([url, init]) => {
+    const data = new URLSearchParams(init.body).get("data")!;
+    const bin = atob(data);
+    const bytes = Uint8Array.from(bin, (c) => c.charCodeAt(0));
+    return { url, ...JSON.parse(new TextDecoder().decode(bytes)) };
+  });
+}
+
+describe("handleInstalled (background)", () => {
+  let mem: Record<string, any>;
+  let fetchMock: ReturnType<typeof vi.fn>;
+  let manifest: Record<string, any>;
+  const tabsCreate = vi.fn(async (_: any) => ({}));
+
+  beforeEach(() => {
+    mem = memoryChromeStorage();
+    manifest = { version: "1.18.0", update_url: "https://clients2.google.com/" };
+    (chrome.runtime as any).getManifest = () => manifest;
+    (chrome.tabs as any).create = tabsCreate;
+    tabsCreate.mockClear();
+    fetchMock = vi.fn(async () => new Response("1"));
+    vi.stubGlobal("fetch", fetchMock);
+    vi.stubEnv("MIXPANEL_PROJECT_TOKEN", "test-token");
+    vi.stubEnv("MIXPANEL_API_HOST", "https://proxy.test/");
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.unstubAllEnvs();
+  });
+
+  it("on install: creates the install id, opens the welcome page, tracks extension_installed", async () => {
+    await handleInstalled({ reason: "install" } as any);
+
+    const id = mem[ANON_ID_KEY];
+    expect(id).toMatch(UUID_V4);
+
+    expect(tabsCreate).toHaveBeenCalledWith({
+      url: `https://subturtle.app/?install_id=${id}&utm_source=extension&utm_medium=install`,
+    });
+
+    const [e] = sentEvents(fetchMock);
+    expect(e.url).toBe("https://proxy.test/track/?ip=1");
+    expect(e.event).toBe("extension_installed");
+    expect(e.properties).toMatchObject({
+      token: "test-token",
+      distinct_id: id,
+      $device_id: id,
+      app: "chrome-extension",
+      version: "1.18.0",
+      reason: "install",
+    });
+  });
+
+  it("reuses an install id that already exists", async () => {
+    mem[ANON_ID_KEY] = "existing-id";
+    await handleInstalled({ reason: "install" } as any);
+    expect(mem[ANON_ID_KEY]).toBe("existing-id");
+    expect(sentEvents(fetchMock)[0].properties.distinct_id).toBe("existing-id");
+  });
+
+  it("on update: tracks extension_updated with previous_version and opens nothing", async () => {
+    mem[ANON_ID_KEY] = "existing-id";
+    await handleInstalled({ reason: "update", previousVersion: "1.17.0" } as any);
+
+    expect(tabsCreate).not.toHaveBeenCalled();
+    const [e] = sentEvents(fetchMock);
+    expect(e.event).toBe("extension_updated");
+    expect(e.properties).toMatchObject({
+      distinct_id: "existing-id",
+      previous_version: "1.17.0",
+      version: "1.18.0",
+    });
+  });
+
+  it("ignores a Chrome update", async () => {
+    await handleInstalled({ reason: "chrome_update" } as any);
+    expect(fetchMock).not.toHaveBeenCalled();
+    expect(tabsCreate).not.toHaveBeenCalled();
+  });
+
+  it("an unpacked build still tracks the install but opens no tab", async () => {
+    delete manifest.update_url;
+    await handleInstalled({ reason: "install" } as any);
+    expect(tabsCreate).not.toHaveBeenCalled();
+    expect(sentEvents(fetchMock)[0].event).toBe("extension_installed");
+  });
+
+  it("a failed analytics request does not throw", async () => {
+    fetchMock.mockRejectedValue(new Error("offline"));
+    await expect(handleInstalled({ reason: "install" } as any)).resolves.toBeUndefined();
+    expect(tabsCreate).toHaveBeenCalled();
+  });
+
+  it("builds the welcome URL with encoded params", () => {
+    expect(welcomeUrl("a b")).toBe(
+      "https://subturtle.app/?install_id=a+b&utm_source=extension&utm_medium=install"
+    );
+  });
+});